Plasma catalysis: synergistic effects at the nanoscale. · 2015. 12. 27. · Plasma Catalysis: Synergistic Eﬀects at the Nanoscale Erik C. Neyts,*,§ Kostya (Ken) Ostrikov,†,‡

Plasma Catalysis: Synergistic Effects at the NanoscaleErik C. Neyts,*,§ Kostya (Ken) Ostrikov,†,‡ Mahendra K. Sunkara,∥ and Annemie Bogaerts§

§Department of Chemistry, Research Group PLASMANT, Universiteit Antwerpen, Universiteitsplein 1, 2610 Wilrijk-Antwerp,Belgium†Institute for Future Environments and School of Chemistry, Physics and Mechanical Engineering, Queensland University ofTechnology, Brisbane, Queensland 4000, Australia‡Plasma Nanoscience Laboratories, Manufacturing Flagship, Commonwealth Scientific and Industrial Research Organization, P.O.Box 218, Lindfield, New South Wales 2070, Australia∥Conn Center for Renewable Energy Research and Chemical Engineering, University of Louisville, Louisville, Kentucky 40292,United States

ABSTRACT: Thermal-catalytic gas processing is integral to many current industrialprocesses. Ever-increasing demands on conversion and energy efficiencies are a strongdriving force for the development of alternative approaches. Similarly, synthesis ofseveral functional materials (such as nanowires and nanotubes) demands specialprocessing conditions. Plasma catalysis provides such an alternative, where the catalyticprocess is complemented by the use of plasmas that activate the source gas. Thiscombination is often observed to result in a synergy between plasma and catalyst. ThisReview introduces the current state-of-the-art in plasma catalysis, including numerousexamples where plasma catalysis has demonstrated its benefits or shows future potential,including CO2 conversion, hydrocarbon reforming, synthesis of nanomaterials, ammoniaproduction, and abatement of toxic waste gases. The underlying mechanisms governingthese applications, as resulting from the interaction between the plasma and the catalyst,render the process highly complex, and little is known about the factors leading to theoften-observed synergy. This Review critically examines the catalytic mechanisms relevant to each specific application.

CONTENTS

1. Introduction 134091.1. Scope and Structure of the Review 134091.2. Brief Historic Perspective 13409

2. General Aspects of Plasma Catalysis 134102.1. Chemical Bonding and Thermal Surface

Processes 134102.1.1. Molecular Description of Bonding on

Catalyst Surfaces 134102.1.2. Surface Reactions and Kinetics 13411

2.2. Catalysis 134122.2.1. Definitions and Concepts in Catalysis 134122.2.2. Properties of Nanocatalysts 13413

2.3. Low-Temperature Plasmas 134142.3.1. Basic Properties 134142.3.2. Plasma−Surface Interactions 13414

3. Plasma−Catalyst Interactions and Synergies atthe Nanoscale 134153.1. What is Synergy? 134153.2. Plasma−Catalyst Interactions and Synergy

1: Effects of the Plasma on the Catalyst 134163.2 .1 . P lasma- Induced Morphologica l

Changes in the Catalyst 134163.2.2. Chemical and Electronic Changes in

the Catalyst 134163.2.3. Changes in Surface Processes 13416

3.3. Plasma−Catalyst Interactions and Synergy2: Effects of the Catalyst on the Plasma 13417

3.4. What Possible Synergies Are CurrentlyUnexplored? 13417

3.4.1. Vibrationally Excited Species 134173.4.2. Plasma Photocatalysis 134183.4.3. Collision-Induced Surface Chemistry 13418

4. Contrasting Catalytic, Plasma, and Plasma-Catalytic Processing: CNT Growth and CO2

Conversion 134184.1. Carbon Nanotube Growth 13418

4.1.1. Thermal Growth with a Catalyst 134194.1.2. Plasma Growth without a Catalyst 134194.1.3. Plasma-Catalytic Growth 13420

4.2. CO2 Conversion 134234.2.1. Thermal-Catalytic Conversion 134234.2.2. Plasma Conversion without a Catalyst 134244.2.3. Plasma-Catalytic Conversion 13424

5. Other Current and Future Applications ofNanocatalyst-Based Plasma Catalysis 134275.1. Catalytic Synthesis of Graphene and Re-

lated Nanostructures 134275.2. Catalytic Growth of Inorganic Nanowires 13429

Received: June 18, 2015Published: November 30, 2015

Review

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© 2015 American Chemical Society 13408 DOI: 10.1021/acs.chemrev.5b00362Chem. Rev. 2015, 115, 13408−13446

pubs.acs.org/CR

http://dx.doi.org/10.1021/acs.chemrev.5b00362

5.3. Abatement of Toxic Waste and Air Pollu-tion Control 13432

5.4. Hydrocarbon Reforming 134335.5. Ammonia Production 13435

6. Conclusions and Outlook 13437Author Information 13438

Corresponding Author 13438Notes 13438Biographies 13438

Acknowledgments 13439References 13439

1. INTRODUCTION

Plasma catalysis is an emerging branch of plasma processing,at the interface of a variety of disciplines, including physicalchemistry, material science, nanotechnology, plasma physicsand plasma chemistry, catalysis, and others. In short, itsobjective is to enhance catalytic reactions by virtue of addinga plasma to the reaction cycle. As most catalysts arenanofeatured materials, the specific plasma−nanostructureinteractions may lead to synergistic effects. Until now,however, very little has been understood in terms of thebasic processes taking place.

1.1. Scope and Structure of the Review

This Review is concerned with gas processing by combining aplasma (partially) derived from that gas and a catalyst. Thegoal of such gas processing is either to convert the gas or gasmixture into another gas mixture, as, for instance, in dryreforming of CH4, or to grow some material or structure fromthe precursor gas molecules, as, for instance, in plasma-enhanced catalytic growth of carbon nanotubes (CNTs). Thisis the realm of (mostly) low-temperature plasmas incombination with heterogeneous catalysis. Thus, we shallnot be concerned with high-temperature plasmas, such asfusion plasmas, nor with homogeneous or enzymatic catalysis.Within this scope, we aim to review the current understandingin the field in terms of the basic processes taking place andprovide an explanation of them based on findings in theliterature, rather than collecting a large number of results.We shall commence by reviewing a few basic general

concepts, which will serve as a basis to discuss the observedplasma-catalytic processes and indicate how a low-temperatureplasma and nanofeatures of the catalyst affect these concepts.Where possible, we will mostly focus on molecular-levelphenomena rather than discuss observations at a phenom-enological basis. Readers already familiar with catalysis(sections 2.1 and 2.2) or low-temperature plasmas (section2.3) may skip the concerning sections.Given the need for fundamental insight in this field, we

shall not be overly concerned with more practical issues, suchas the preparation of (nano)catalysts, reactor design, orperformance. Moreover, as we will focus on nanoscalephenomena in catalysis, and how these are affected by theplasma, we will not provide an extensive overview of types ofplasmas, plasma processes, or applications of plasmas.As will be pointed out in sections 4 and 5, plasma catalysis

is already a highly successful approach for small-scalefabrication of expensive materials, including, for instance,CNTs or inorganic nanowires. Indeed, plasma catalysiscurrently still is an expensive process, due to the high energyinvestment and capital costs involved, currently prohibiting

the large-scale production of basic chemicals such as synthesisgas (a mixture of CO and H2), ethylene epoxide, or methanol.However, reducing this energy cost is a very active field ofresearch within the plasma community, and it is exactly thecombination of plasmas with catalysis that may openperspectives for better energy efficiency, in combinationwith the selective production of targeted compounds. Weshall return to this issue in section 4.2.

1.2. Brief Historic Perspective

Catalysis is already an old and very mature field in chemistry.1

The first report came from Johann Wolfgang Dobereiner in1823, stating that “finely divided platinum powder causeshydrogen gas to react with oxygen gas by mere contact towater whereby the platinum itself is not altered”, a processthat in 1835 was termed “catalysis” by Jons Jacob Berzelius.Around 1900, Wilhelm Ostwald defined catalysis in terms ofchemical kinetics: “A catalyst is a substance which affects therate of a chemical reaction without being part of its endproducts”. A few years later, in 1909, Ostwald was awardedthe Nobel Prize in Chemistry for his contributions tocatalysis. Some extremely important reactions also date backto that period. For instance, Fritz Haber realized the nitrogenfixation reaction N2 + 3H2 → 2NH3 in 1909 on an osmiumcatalyst (see also section 5.5), and Fischer and Tropschreported on the conversion of synthesis gas (a mixture of COand H2, commonly termed syngas) in hydrocarbons in 1924(see also section 5.4). Nowadays, some 80% of all industrialprocesses are based on catalysis, which amounts to a value forcatalysis-produced goods of about $20 trillion yearly world-wide.2

What was not realized as such in the early days of catalysisis that a large surface area is crucial. This determines thenumber of active catalytic sites and is, thus, crucial for theoverall performance of the catalyst. To quote Ertl,1 “In fact,catalysis has been a nanotechnology long before this term wasintroduced.” We shall elaborate on the importance of catalystnanofeatures in section 2.2.2.Plasma science is also an old field; the first gas discharge

was created by Francis Hauksbee in 1705, essentially bycharging and discharging an evacuated sphere containing asmall amount of mercury. The first industrial device was a so-called silent discharge or dielectric barrier discharge used forproducing ozone, in 1857 by Siemens. The term “plasma” forthis kind of gas discharge was coined by Langmuir in 1927.3

Gas discharges or low-temperature plasmas nowadays are usedin a large number of applications.4,5 Many current plasmaprocessing and plasma applications rely on a limited numberof fundamental plasma−surface interaction processes, de-scribed in section 2.3.2.Whereas catalysis is most often conducted as a thermal

process, plasma-based catalysis is gaining in interest andpopularity. One of the earliest reports on plasma catalysis isthe 1976 U.S. patent by Henis for NOx removal.

6 The plasmaadds a number of means to control the process, potentiallyallowing a more efficient process. As we will describe in detailbelow, the plasma enables one to modify both the feedstock(section 2.3.1), by virtue of the occurring plasma chemistry inthe gas phase, and the catalyst (section 3.2), by virtue of theplasma/surface interactions. The catalytic process in plasmacatalysis is thus determined by both the “bare” catalyticsystem and the nanofeatures of the catalyst, as well as by themodification of gas phase and catalyst by the plasma. This

Chemical Reviews Review

DOI: 10.1021/acs.chemrev.5b00362Chem. Rev. 2015, 115, 13408−13446

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interaction between the plasma and the nanofeatures of thecatalyst may lead to sometimes unexpected benefits orsynergy. A variety of synergistic effects have been observed,examples of which will be described in sections 3, 4, and 5.

2. GENERAL ASPECTS OF PLASMA CATALYSIS

2.1. Chemical Bonding and Thermal Surface Processes

At the fundamental level, catalysis is determined by theprecise nature of the adsorbate/catalyst interaction.1 More-over, on the microscopic level the catalyst is nanofeatured, i.e.,it is either composed of nanoparticles, or it containsnanofeatures such as high nanoscale roughness or nanopores.It is therefore natural to consider the atomic/molecular leveldetails of catalysis as fundamental to the process. We shallnow first concentrate on these molecular-level details. First,we shall review the basics of bonding of adsorbates to metalsurfaces at the molecular level (section 2.1.1) and surfacereactions and kinetics (section 2.1.2), to sketch the conceptualframework relevant for many catalysis processes, followed by ashort discussion on zeolites and metal oxides as used inplasma catalysis. Subsequently, in section 2.2.1, we describebasic definitions and concepts in thermal catalysis, and insection 2.2.2, we continue by reviewing the properties ofnanocatalyst particles and the influence of the nanoscalefeatures of the nanocatalyst on the catalysis process. As wewill see, these concepts are of major importance for a widevariety of plasma-catalytic processes, including the growth ofCNTs, graphene, and inorganic nanowires, abatement of toxicwaste, CO2 conversion, hydrocarbon reforming, ammoniaproduction, and more, as will be described in sections 4 and5.2.1.1. Molecular Description of Bonding on Catalyst

Surfaces. Transition Metal Surfaces. A large portion ofactive catalysts used in plasma catalysis are transition metals.The adsorbates are very often hydrocarbons or functionalizedhydrocarbons, as, for instance, in CNT or graphene growth(sections 4.1 and 5.1), dry reforming of methane (DRM)(section 4.2), or volatile organic compound (VOC) abate-ment (section 5.3). The interaction of hydrocarbons withtransition metals is determined, to a large extent, by theposition of the metal in the periodic table.7−9 In general, lessnoble metals interact stronger with the atoms than withundissociated molecules, which leads to molecular dissocia-tion. More noble metals, on the other hand, show theopposite trend, which preserves the molecules intact. Thesefeatures are important for catalytic action and are affected bythe plasma−surface interactions. The latter will be discussedin more details in section 3. Here, we first explain themicroscopic mechanisms of adsorbate−adsorbent interactions.In the case of an atomic adsorbate, the (vertically oriented)

pz orbital of the carbon (or oxygen or nitrogen) atominteracts with symmetric surface orbitals like the dz2 orbitals orsymmetric combinations of s orbitals, typically leading to highcoordination. The coupling of the adsorbate to the broadmetal s/p states leads to a shift and broadening of theadsorbate states, which is essentially the same for all transitionmetals.10,11 Additionally, the asymmetric px and py orbitalsmay interact with asymmetric surface orbitals like the dxz orasymmetric combinations of s orbitals. This coupling is largelyresponsible for the difference between the different transitionmetals.

The coupling between the adsorbate states and the metal dstates typically gives rise to bonding and antibonding states,11

as shown in Figure 1. In more noble metals such as Cu, the

antibonding states are below the Fermi level and thus areoccupied. This decreases the overall adsorbate−metal bondinginteraction. In contrast, in less noble metals such as Ni or Fe,these antibonding states are above the Fermi level and thusare unoccupied. Bonding of adsorbates is stronger, therefore,at non-noble metals than it is at noble metals.12,13 Therefore,the metal−adsorbate bond becomes stronger when moving tothe left in the periodic table.When considering molecular adsorbates, there are usually

more adsorbate valence states to consider than in the case ofatomic adsorbates, although the overall picture remains thesame. Consider for instance, CO binding to a transition metal.The CO valence states to consider are the filled 5σ and theempty double degenerate 2π* states, as shown in Figure 2.The interaction with the metallic s states merely results in adownshift and broadening of both the 5σ and 2π* states, aswas the case for atomic adsorption. Interaction with the dstates, on the other hand, leads to the formation of bondingand antibonding states below and above the original states,12

respectively, as indicated in Figure 1.The emerging picture is that the CO−metal bond consists

of electron donation from the 5σ state to the metal, as well asback-donation from the metal to the 2π* state. Thecontribution of the 5σ stateelectron donationto bondingis negligible or even negative: both the bonding andantibonding orbitals will be populated, as they lie below theFermi level. The contribution of the back-donation, incontrast, determines the bonding interaction. Before bonding,the adsorbate 2π* states are above the Fermi level. Uponbonding, they shift partially below the Fermi level, leading tosignificant bonding. As we now move toward the left in theperiodic table, starting from noble metals, the d-band centermoves up, and more antibonding states move through theFermi level and become empty, thus strengthening the bond.The effect, however, is weaker for molecules than it is foratoms.

Figure 1. Local density of states projected onto an adsorbate stateinteracting with the d bands at a surface. The strength of theadsorbate−surface coupling matrix element V is kept fixed as thecenter of the d bands (εd) is shifted up toward the Fermi energy (εF= 0) and the width W of the d bands is decreased to keep thenumber of electrons in the bands constant. As εd shifts up, theantibonding states are emptied above εF and the bond becomesstronger. Reproduced with permission from ref 12. Copyright 1998The Royal Society of Chemistry.



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The more noble metals, which show the weakestinteractions, therefore show a stronger bonding to moleculesthan to the corresponding atoms (i.e., CO will bind strongerthan the separate C and O atoms, such that moleculardissociation is not favored), but when moving to the left inthe periodic table, the increase in metal−adsorbate bondstrength is greater in the case of atomic adsorbates than in thecase of molecular adsorbates, such that lesser noble metalsshow stronger bonding to atoms than to the correspondingmolecule (i.e., separate C and O atoms will bind morestrongly to the metal than the CO molecule, such thatmolecular dissociation is favored).Transition Metal Oxide Surfaces. Oxides of transition

metals are used for both thermal catalysis and plasmacatalysis. The three most widely used metal oxide catalystsin plasma catalysis are CoOx, MnOx, and TiO2. Metal oxidenanostructures are characterized by their redox and acid−baseproperties.14 Some metal oxides, like TiO2, are also widelyused thanks to their photocatalytic activity. Metal oxides showa variability in both structure and oxidation state, form mixedvalence compounds, and often exist as nonstoichiometriccompounds, which further adds to their catalytic proper-ties.15−17 In plasma catalysis, metal oxides are often used assupport material for active transition metal catalysts, next toother typical support materials such as carbons, SiO2, andzeolites.15

The acid−base and redox properties of metal oxides aredetermined by their ability to accept electron density and thusto be reduced, leading to the generation of Brønsted acid sitesor stabilizing the cationic transition state.14,18 Such reductionprocesses are favored by low-lying LUMO orbitals, and thisability is size-dependent. Indeed, the relative acidity andbasicity of a particular surface site is determined by the localcoordination of the site atoms. Hence, factors such as size,geometry, and defects greatly influence the catalytic andplasma-catalytic behavior.Metal oxides show a wide distribution in work function,

from ∼2 eV for ZrO2 to ∼7 eV for V2O5. As will be discussedin section 3.2.2, the plasma may affect the catalyst workfunction. Depending on the position of the Fermi level, andthe alignment with the HOMO−LUMO gap of the adsorbent,the adsorbing molecule may either remain neutral uponadsorption or become charged.19 Specifically, when the Fermilevel lies within the HOMO−LUMO gap, the neutral

molecule is thermodynamically most stable. When on theother hand the Fermi energy becomes higher than themolecules’ ionization energy, the ionized form of the moleculewill be more stable.19 Also, the precise position of the Fermilevel is determined by the presence of oxygen vacancies orinterstitials, in addition to general factors such as strain,curvature, facetting, etc. affecting their properties. Forinstance, as the metal oxide domains become smaller, theelectron density becomes less delocalized and the band gapincreases.18

The essential process in metal oxide redox catalysis is thedonation of surface lattice oxide ions to an adsorbedreductant, as well as the subsequent uptake of oxygen fromthe gas phase to reoxidize the metal oxide. This mechanism isknown as the Mars and Van Krevelen mechanism. In additionto the direct oxidation by oxygen species originating from theplasma, the Mars and Van Krevelen mechanism is probablyessential in catalytic oxidative degradation of VOCs.20

Zeolites. Zeolites are often described as solid acids, whosecatalytic function is mainly determined through their surfaceacidity and shape selectivity.21,22 In the context of plasmacatalysis, zeolites have been used both as catalysts and ascatalyst-support materials.23−27

A distinguishing feature of zeolites is their unique andsharply defined geometry. As a result, zeolites often exhibitshape selectivity, defined as the dependence of theheterogeneously catalyzed reaction on the pore size and/orpore architecture.21,28 In the context of plasma catalysis,however, the importance of the micro- and nanofeaturedzeolite structure is the possible ability to generate a plasmanear or perhaps even inside the pores.29,30 Because of thesmall dimensions, the electric field near these pores may bevery high, leading to plasma-catalytic mechanisms notavailable in thermal catalysis. The effects and importance ofcatalyst-induced electric field enhancement will be discussedin section 3.3.

2.1.2. Surface Reactions and Kinetics. Bond Breakingand Bond Formation. Activated processes are characterizedby the activation barrier Ea separating products from reactants.In a thermal process, the energy required to overcome thisbarrier is supplied by the thermal energy kBT.In plasma catalysis, the plasma itself may affect the energy

barrier of the process in several ways, e.g., by altering thereactants either chemically by creating radicals, by providing

Figure 2. (a) Formation of the CO molecular orbitals from the atomic 2s and 2p oxygen and carbon atoms, showing the highest occupiedmolecular orbital (HOMO) 5σ and the lowest unoccupied moleculary orbital (LUMO) 2π* orbitals; (b) donation and back-donation in COinteracting with the metal d orbitals (here shown for copper).



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some of the energy required for overcoming the activationbarrier by excitation (see sections 3.2.3 and 3.4.1), or bymodifying the size, structure, and morphology of the catalyst,thereby also changing its properties and thus its activity.These factors will be described in more detail in section 3.2.Note that besides the kinetics of the reaction also thethermodynamics of the process are modified by both thenanofeatures of the catalyst and the plasma.31

Early and Late Barriers. The bond formation and bondbreaking may be understood in generally the same terms asthose used to describe adsorption and desorption.11,32,33

Conceptually, there are two types of surface reaction barriers(Figure 3): so-called early-barrier reactions and late-barrier

reactions. In early-barrier reactions, the transition stateresembles the initial configuration of the reactants, whereasin a late-barrier reaction, the transition state resembles thefinal configuration, resembling the products.A typical example of an early-barrier reaction is dissociative

adsorption of H2 on transition metal surfaces.34 Early-barrierreactions are usually enhanced by translational energy insteadof vibrational excitation. In this case, the barrier isencountered before the bond has noticeably elongated.Typical examples of late-barrier reactions are the dissociativeadsorption of H2 on noble metals or the catalytic dissociationof CH4 on transition metal surfaces.35−37 The latter process isof interest in plasma-catalytic dry reforming. In this case, oneof the hydrogen atoms nearly completely separates from themethane molecule at the transition state, which resembles thefinal products in which both the hydrogen atom and the CH3radical are bonded to the metal surface.We may now also understand the difference in reactivity

from one metal to another. As described above, the higher thed-band center (i.e., moving toward the left in the periodictable), the more bonding orbitals will be below the Fermilevel, the stronger the bonds will be between adsorbate andthe metal, and the more prone the adsorbate will be todissociation. Hence, the lesser noble metals show a higherreactivity and, correspondingly, a lower activation barrier. Thisdetermines the reactivity in early-barrier reactions. In late-barrier reactions, the transition state resembles the products.The differences in transition state energies thus also closelyfollow the differences in adsorbed products.2.2. Catalysis

2.2.1. Definitions and Concepts in Catalysis. Catalysisis defined as the process in which the rate of a reaction isincreased without changing the overall standard Gibbs energy

change. This rate increase is brought about by the use of acatalyst, which itself is regenerated after each reaction cycle.A number of measures are used to determine the catalyst

performance and activity. The overall catalyst activity ismeasured by the conversion, which is the number of moles ofreactant converted into products divided by the number ofmoles of reactant fed. The selectivity for a given product isdefined as the conversion of the reactant(s) into the desiredproduct divided by the total conversion of the reactant(s). Itis thus roughly equal to the ratio of the reaction rate formingthat product over the total rate. Also, for a given set ofreaction conditions, the selectivity is often found to bedependent on the conversion.38−40 The selectivity isdetermined by the mechanisms of the competing reactionsand by the surface state of the catalyst, as will be explainedlater. The yield is defined as the number of moles of productdivided by the number of moles of reactant fed, or,equivalently, the product of the total conversion and theaverage selectivity. Finally, another measure for the activity isthe turnaround frequency, or TOF, and is defined as thenumber of times that the overall catalytic reaction takes placeper catalytic site and per unit of time for a fixed set ofreaction conditions.A central concept in catalysis is the Sabatier principle,

which states that optimum activity is obtained for an optimuminteraction energy between the adsorbates and the catalyst.When the reactants reach the surface, they need to adsorband bind sufficiently strongly to the catalyst such that they donot immediately desorb into the gas phase again. Theirprobability to react is related to their lifetime. After allrequired elementary reactions have taken place, the formedproducts need to desorb from the surface. Thus, theseproducts may not be bound too strongly to the surface, as thiswould prevent them from desorbing. The optimum reactantadsorption is reflected by so-called volcano plots, an exampleof which is shown in Figure 4.41

Consider, for instance, the adsorption of CO on threetypical metal surfaces: Co, Ni, and Cu. This is a critical stepin, e.g., plasma DRM. The largest heat of adsorption for COis found on Co, i.e., 110 kJ/mol. Of these three metals, Cuhas the lowest d-band center, and Co has the highest. Thus,Co is most reactive and shows the strongest binding, both

Figure 3. Calculated position of the transition state for H2dissociation on metallic substrates: (left) early barrier; (right) latebarrier. Reprinted with permission from ref 34. Copyright 2003Elsevier.

Figure 4. Typical volcano plot, here shown for formic acidformation. Reproduced with permission from ref 41. Copyright2014 The Royal Society of Chemistry.



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toward the molecule and toward the separate atoms (seesection 2.1). As the molecular binding energy of CO on Cu isless than the CO bond energy, CO will not dissociate on Cu.The heat of adsorption of CO on Ni is ∼95 kJ/mol, and onCu it is 80 kJ/mol. This small difference in adsorption energybetween Ni and Cu is magnified when the moleculedissociates, because after dissociation the valence electronsof the CO molecule are no longer tied to molecular orbitalsbut are freely available. Consequently, the dissociation processis exothermic on Co, thermoneutral on Ni, and endothermicon Cu. Such simple arguments explain, for instance, thedifference in selectivity between these metals for theconversion of synthesis gas: while Cu is an active componentfor methanol synthesis catalysts, Co is a Fischer−Tropschcatalyst producing higher hydrocarbons.The Sabatier principle and the typical volcano curves are a

direct result of the above.11 Indeed, moving toward the left inthe periodic table gives a lower activation barrier and higherreactivity but also stronger bonding to the surface and, thus,less free active surface and less desorption. What is less clearlyemphasized in the literature is that volcano plots are alsodependent on the catalyst size, as adsorbate−catalystinteractions are indeed size-dependent. Therefore, we shallnow briefly review nanocatalyst properties.2.2.2. Properties of Nanocatalysts. The nanoscale

feature dimensions of catalysts are responsible for the verydifferent properties of nanocatalysts compared to their bulkcounterparts and, in fact, for their actual use as catalysts. Letus discuss the properties of nanocatalysts that are mostrelevant to plasma nanocatalysis. A nanoparticle is defined as aparticle containing fewer than ∼106 atoms, as above thisnumber the properties resemble those of the bulk. Thisnumber corresponds to a diameter of less than ∼100 nm.42,43

The activity of nanoparticles is determined, to a largeextent, by their size, composition, morphology, and interactionwith the support. The origin of this dependence resides in thesize-dependence of their geometrical and electronic struc-ture.44 Given that the surface-to-volume ratio increases withdecreasing particle size, and that the chemical reactivity of aheterogeneous catalyst is proportional to this ratio, smallnanoparticles are generally the more-efficient catalysts.45

Detailed investigations have revealed, however, that therelationship between size and reactivity is not alwaysmonotonic. A clear example is the size-dependence ofadsorption energies on nanoparticles. This size-dependenceof the nanocatalyst chemical reactivity has been demonstratedexperimentally by Campbell et al. (as shown in Figure 5a)46,47

and theoretically by Yudanov et al. (Figure 5b).48 It wasshown by density functional theory (DFT) calculations thatthe CO adsorption on Pd-nanoclusters is weakest (lowestadsorption energy) for clusters containing 30−50 atoms.Below and above this size range, the CO adsorption isstronger. Below this size range, the interaction is strongerbecause of the decreasing energy gap between the 2π* lowestunoccupied molecular orbital (LUMO) of the CO moleculeand the energies of the d levels of the metal. Above this size,on the other hand, the interaction energy increases due to adecrease in lattice contraction with increasing particle size. Asa result of these size-dependent interactions, volcano plots arealso size-dependent.The electronic structure of nanoparticles is not composed

of complete bands as in the case of macroscale solids but isintermediate between atoms and molecules on the one hand

and solids on the other hand. Schematically, this may berepresented as shown in Figure 6.49 Indeed, for nanoparticles

smaller than ∼10 nm, the electronic, optical, and magneticproperties of the material change due to quantum mechanicaleffects. Specifically, when the de Broglie wavelength of theelectrons becomes comparable to the system size, the particleselectronically behave as zero-dimensional.45 The de Brogliewavelength of a 1 eV electron with rest mass energy of 0.511MeV is 1.23 nm, which is a typical nanocatalyst particle size.Nanofeatures of catalysts typically show a wide variety of

surface structures, possibly containing various crystal facets,amorphous local surfaces, and defects. This is especiallyimportant because both the interaction energy and thereaction mechanism are determined by the coordinationnumber of the atoms involved. Generally, lower-coordinatedsurfaces are more reactive than densely packed surfaces, andhence catalysts usually come in the form of nanoparticles.The melting point of nanoparticles is another well-known

size-dependent property of nanocatalysts, affecting theircatalytic behavior. Melting of nanoparticles is described bythe Gibbs−Thomson equation,

σρ

Δ =Δ

TT

Hr2 M SL

S (1)

Figure 5. (a) Size dependence of the heat of adsorption of Ag onAg. Reprinted with permission from ref 47. Copyright 2013American Chemical Society. (b) Size dependenceof the heat ofadsorption of CO on Pd. Reprinted with permission from ref 48.Copyright 2011 American Chemical Society.

Figure 6. Electron levels of atom, cluster, nanoparticle, and bulkmetal. Reproduced with permission from ref 49. Copyright 2013John Wiley and Sons.



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expressing the depression of the melting temperature of smallparticles relative to the bulk material, where TM is the bulkmelting temperature, σSL is the solid−liquid interfacial energy,ρS is the density of the solid, ΔH is the melting enthalpy, andr is the radius of the particle. A variety of experimental50−52

and theoretical works53−57 have indeed demonstrated thismelting-point depression. This is especially important forprocesses taking place at elevated temperatures close to thesize-dependent melting temperature, as is the case in CNTgrowth, for instance. Depending on the exact processconditions, the nanocatalyst may exist in either the solid orthe liquid state, and consequently, a small change intemperature may induce a change in process mechanism.This has been observed in metal-catalyzed CNT growth,where carbon transport may occur either by surface diffusionat low temperature (solid nanocatalyst) or by bulk diffusion athigher temperature (liquid nanocatalyst).The most important effect on the catalytic behavior of a

material seems to be the occurrence and availability ofundercoordinated surface atoms, including face atoms (terraceatoms), corner atoms, edge atoms, and kink atoms.58,59 Othereffects, including the interaction with the support and chargetransfer, also contribute but to a lesser extent. DFTcalculations point out that changes in metal coordinationlead to significant changes in catalytic behavior.60

To summarize, the catalytic activity of nanoparticles isdetermined by their size, facetting, presence of steps, defectsand other “special sites”, strain, oxidation state, and supportmaterial used. In plasma catalysis, all of these factors areinfluenced by the plasma, thus affecting the catalytic process.31

2.3. Low-Temperature Plasmas

2.3.1. Basic Properties. A plasma or gas discharge is apartially ionized gas, consisting of electrons, various ions, andneutral species (molecules, radicals, and excited species),which can all interact with each other, giving rise to a “rich”and reactive plasma environment. A key characteristic of suchplasmas is their far-from-equilibrium state at relatively lowtemperatures, typically in the range 300−1000 K. Thiscombination of reactivity, far-from-equilibrium state, andlow-temperature operation enables plasmas to be used formany complex applications, including catalysis. Plasmareactivity in turn increases reactivity at surfaces in contactwith the plasma. As a result, low-temperature plasmas showsuperior characteristics that allow the production of structuresand induce processes at surfaces more efficiently and withmore control than is possible with traditional thermalmethods. Here we introduce the various aspects of low-temperature plasmas and plasma−surface interactions ofparticular relevance to plasma nanocatalysis.Low-temperature plasmas used in plasma catalysis are

characterized by the occurrence of various temperature scalesand energy distributions for neutrals, ions, and electrons, ahigh chemical reactivity, and their lack of selectivity in termsof product formation. Depending on how the power iscoupled into the plasma, a variety of different plasmas may beobtained, including direct current (dc), capacitively coupledradio frequency (RF), and inductively coupled RF sources,microwave discharges, dielectric barrier discharges, glidingarcs, plasma jets and plasma torches, etc.4,5

Structure of the Plasma. When an electric field ofsufficient magnitude is applied to a gas, the gas will (partially)break down in ions and electrons. Because of the much higher

mobility of electrons compared to the ions, all exposedsurfaces will attain a negative charge, which sets up an electricfield, slowing down the electrons and accelerating the positiveions. This in turn establishes a dynamic equilibrium, withequal fluxes of electrons and ions. As a result, a region infront of the surface develops that is characterized by an excesspositive charge. This region is called the sheath. The sheath isextremely important in plasma-processing applications, as itdetermines the fluxes and energies of the (charged) speciesreaching the surface. Also the neutral gas is affected in thesheath, as the modified energy distributions of the electronsand ions influence the reactions taking place. Thus, theplasma−surface interactions are determined, to a large extent,by the electric-field distribution in the plasma and especially inthe sheath, as well as to closely related plasma characteristicssuch as the power coupled into the plasma, the gas pressure,and the chemical composition of the plasma.

Energy Distributions in Plasmas and Reactivity. Thechemical composition of the plasma is determined by theoccurring reactions, which are in turn determined to a largeextent by the energies of the electrons, ions, and neutrals. Theelectron energy distribution function (EEDF) is stronglydependent on the precise electric field distribution and gascomposition, as well as on the discharge type.The EEDF might be a simple Maxwellian distribution or

might show a more complex behavior such as the bi-Maxwellian distribution or Druyvenstein distribution. The ionenergy distribution function (IEDF) is typically not far fromMaxwellian,5 as the ions are typically too heavy to be stronglyaccelerated by the electric field. The neutral species aretypically showing a simple Maxwellian distribution, as they arenot (directly) influenced by the electric field. Gas heating,however, may occur through collisions with (mainly) theenergetic ions.Vibrationally excited species are of special importance in

plasma catalysis, because they may strongly influence theoccurring plasma−surface interactions (see section 3.4.1).Moreover, the energetic electrons induce reactions creatinghighly reactive radicals, which can react further into newmolecules. Hence, even reactions that are strongly endother-mic and that require high temperatures to proceed underthermal reaction conditions can occur in a plasma atsufficiently high rates at room temperature.

2.3.2. Plasma−Surface Interactions. Because of theirpeculiar properties, plasmas interact with surfaces differentlyfrom neutral gases. These interactions are determined by thefluxes and energies of all species involved, and they induceprocesses such as sputtering, etching, heating and formation ofhot spots, charging, deposition, implantation, and photonirradiation.61 These processes affect the plasma-catalyticprocess and are briefly discussed here.

Etching and Sputtering. Etching is the chemical removalof surface material by formation of volatile compounds uponimpinging particles reacting at the surface. A typical examplein plasma catalysis is etching of amorphous carbon withhydrogen in plasma-catalytic growth of carbon nanotubes andother nanocarbons.62−64

Sputtering is the physical removal of surface material byimpact of energetic particles, typically ions. The impact of theion on the surface results in a collision cascade in the topsurface layers, leading to the ejection of material. It has beenshown both theoretically65,66 and experimentally67−70 that



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sputtering can be a crucial factor in plasma-catalytic growth ofCNTs.Heating. Plasmas can heat up surfaces they are in contact

with by the impact of charge carriers, photons, metastable andexcited neutrals, and fast ground-state neutrals, as well asthrough exothermic surface reactions. Locally intense electricfields induced by the strong local curvature of the nano-particles contribute to the heating. The heating effect dependson the power coupled into the plasma and may lead to theformation of so-called “hot spots” that influence catalyticprocesses.71−74 Plasma heating may reduce or even eliminatethe required external heating. When catalyst nanoparticles ornanofeatures are subjected to this heating, they can meltbecause of the Gibbs−Thomson effect (see section 2.2.2),resulting in, e.g., a different solubility.56,75,76 Moreover, theplasma heating is a localized effect where energy ispredominantly delivered to the topmost atomic layers ofcatalyst nanoparticles (CNPs), where nanoscale plasma−surface interactions are most intense. This effect is of greatimportance for, e.g., plasma-enhanced metal-catalyzed CNTgrowth, where ion bombardment stimulates the nanotubenucleation process.65,66 This will be further elaborated insection 4.1.3.Charging and Charge Transfer. Continuous electron and

ion deposition leads to surface charging, which affects thecatalytic function, especially in the case of nanocatalysts.Indeed, charging is a size-dependent effect, and single-electron-transfer and charge fluctuations come into play atsmall nanoparticle sizes. Therefore, charge transfer to andfrom the plasma affects the charge on catalyst nanoparticlesurfaces, which is determined by the dynamic balance betweenthe electron and ion currents onto the surface. Thedynamically varying electric charge in turn affects the catalyticfunction of nanocatalysts. An example of charge transfer inplasma-catalytic growth of CNTs is provided in section 4.1.3.Deposition and Implantation. Plasmas are typically very

reactive, due to the effective creation of radicals thateffectively stick to the surface, thus making plasmasparticularly suitable for deposition of nanomaterials such asgraphene, CNTs, or inorganic metal oxide nanowires asdiscussed later (see sections 5.1, 4.1, and 5.2, respectively).Neutral nonradical molecules created in the plasma alsocontribute to the deposition process. However, the stickingcoefficients that quantify their attachment to the surface aretypically lower compared to the radical species.Ions with sufficient energies may penetrate the surface of

the substrate and become implanted. This effect may changethe surface condition and electronic structure of the catalyst,thereby affecting its catalytic performance. For instance,atomic-scale simulations predict that the CNT growth modemay be modified by allowing energetic ions to impinge on thegrowing structure.77,78 On the other hand, experiments showthat ion bombardment modifies the catalyst dispersion.79

Photon Irradiation. Gas discharges create a significantnumber of electronically excited species, which may decay to alower-energy state by emitting photons. These photons maybe advantageous in plasma catalysis, as they may activatephotocatalysts, such as anatase TiO2 (see section 5.3).However, the photon flux generated by low-temperature

plasmas should be sufficiently high to play a significant role inphotocatalytic processes.80,81 The arising opportunities arepresently underexplored, and we will return to this issue insection 3.4.

3. PLASMA−CATALYST INTERACTIONS ANDSYNERGIES AT THE NANOSCALE

When the effects of nanoscale features described above arecombined with the presence of a plasma, interestinginteractions appear, yielding improved process results interms of efficiency, rate, yield, or selectivity. This phenom-enon is often termed synergy. We shall now describe theorigins of this synergistic effect.

3.1. What is Synergy?

In the context of plasma catalysis, we can define synergy asthe surplus effect of combining the plasma with a catalyst, i.e.,the effect of combining the plasma with the catalyst is greaterthan the sum of their individual effects. Synergism in plasmacatalysis is a complex phenomenon, originating from theinterplay between the various plasma−catalyst interactionprocesses.61,82−85 Conceptually, we can break down theinteraction in two parts. First, both the plasma and thecatalyst, independent from each other, affect the surfaceprocesses. The plasma does so by establishing an electric fieldand by modifying the gas composition, resulting in thedelivery of a variety of reactive species, ions, electrons, andphotons to the surface where the plasma−catalyst interactionstake place. The catalyst will also affect the surface reactions,by lowering the activation barrier for certain reactions.Second, the plasma and catalyst show some interdependence,as the plasma affects the catalyst properties and the catalystaffects the plasma properties. For instance, the plasma canmodify the catalyst morphology or work function, modifyingthe catalyst operation. Vice versa, catalyst properties such asthe dielectric constant or its morphology affect plasmaproperties near the surface such as the electric-fielddistribution and the electron-energy distribution. Thiscomplex interdependence is graphically represented in Figure7.

An important parameter in plasma catalysis, often differ-entiating the operating conditions from thermal catalysis, isthe system pressure. Thermal catalysis operates in a widerange of pressures, including reduced pressures for, e.g., thegrowth of nanomaterials such as CNTs or graphene and highpressures, as in the case of, e.g., ammonia production. Thecurrent tendency in plasma catalysis, in contrast, is to work atatmospheric pressure. Relative to a low-pressure setup, thiseliminates the requirement of using expensive vacuum

Figure 7. Complex interdependence of effects of plasma on catalystand catalyst on plasma. Reprinted with permission from ref 85.Copyright 2011 Elsevier.



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equipment and it reduces the influence of damaging ionbombardment. High pressures (i.e., above atmosphericpressure) at near-room temperature, on the other hand, arenot feasible in plasma catalysis, because the high-frequencygas-phase collisions would thermalize all species, extinguishingthe plasma.

3.2. Plasma−Catalyst Interactions and Synergy 1: Effectsof the Plasma on the Catalyst

A significant number of effects of the plasma on the catalysthave been reported and are discussed in this section.Importantly, these effects and induced changes are closelyrelated. For instance, a change in morphological propertiestypically leads to a modification of the electronic properties ofthe catalyst, which in turn leads to a change in its chemicalproperties. The division among the various listed effects is,therefore, somewhat arbitrary.3.2.1. Plasma-Induced Morphological Changes in the

Catalyst. The application of plasma has been observed tolead to a higher catalyst dispersion and, hence, to a largeractive surface area.86−88 This can be attributed in part to thelower process temperatures that can be employed in plasma-catalysis systems, leading to less agglomeration and sinteringof catalyst particles and, hence, to a higher dispersion.Moreover, bombardment of the catalyst surface by chargedparticles may contribute to an enhanced dispersion. Shang etal., for instance, demonstrated that the dispersion of the Ni/γ-Al2O3 catalyst more than doubled from 14.7% when preparedthermally to 30.6% when prepared by a plasma-enhancedmethod.79

Application of the plasma may also lead to significantchanges in surface structure and morphology, due to the factthat the catalyst surface is subject to large fluxes of plasmaspecies, including energetic ions, radicals, and electrons, whichmay induce a change in surface facetting and oxidation stateand, hence, stability and structure of the surface. For instance,Guo et al. demonstrated a change in morphology of theirmanganese oxide catalyst when exposed to a dielectric barrierdischarge (DBD) plasma.89 It was found that the granularityof the catalyst features decreased upon plasma exposure, alongwith an increase in catalyst dispersion. Hence, the specific areaof the catalyst was enlarged and the number of special sites(such as vacancies, corner atoms, edges, etc.) at the catalystsurface increased, leading to a plasma-enhanced reactivity.89

3.2.2. Chemical and Electronic Changes in theCatalyst. Change in Oxidation State. Upon exposure ofthe catalyst to the plasma, the oxidation state of the catalystmay change. For instance, the change in catalyst morphologyobserved by Guo et al., as described earlier,89 wasaccompanied by a decrease in oxidation state of themanganese from Mn(III) to Mn(II,III). Similarly, thereduction of NiO to metallic Ni was observed by Gallon etal.,90 Tu et al.,91 and others79,92 upon plasma exposure, in thecontext of plasma DRM.In contrast, an increase in the oxidation state of a

manganese oxide catalyst from Mn(IV) to Mn(V) wasfound by Demidyuk and Whitehead, during plasma-catalyticdecomposition of toluene.93 This increase in oxidation statewas attributed to the interaction with plasma-generated Oatoms.Change in Catalyst Work Function. The work function of

a catalyst is highly sensitive to its precise surface condition,including, for instance, its morphology, the occurrence of

contaminations, and surface reactions. Because the plasmastrongly affects the catalyst surface condition, it is clear thatthe catalyst work function will be influenced. Additionally, thepresence of a voltage or current may alter the work function.Hence, the catalytic activity and selectivity is governed in partby the plasma-induced modification of the catalyst workfunction.

3.2.3. Changes in Surface Processes. Reduced CokeFormation and Poisoning. An important issue in catalysis ispreventing or limiting catalyst poisoning and coke formation.This may be achieved by combining the catalyst with aplasma. An interesting example of such a plasma/catalystsynergism is the NH3 decomposition for fuel cellapplications.94 Whereas NH3 conversion was only 7.4% and7.8% in the case of Fe-catalyst alone and DBD plasma alone,respectively, a conversion of 99.9% was obtained whencombining both. This spectacular conversion increase wasattributed to plasma-induced prevention of nitrogen poison-ing, by N2 desorption initiated by surface-adsorbed N atomsrecombining with activated plasma species such as metastableNH3 molecules and NH radicals. However, we note that thereactor was operated using an excessive input power of about3600 J/L, which would cause too high an energy cost fromthe practical point of view.

Modification of the Reaction Pathways. Besides prevent-ing catalyst poisoning and coke formation, the application ofthe plasma also entails a modification of the reactionpathways. As is clear from the above, the interaction betweenplasmas and. In the thermal process, Wang et al. found thatdesorbing N2 molecules are formed by surface recombinationof two N atoms, i.e., following a Langmuir−Hinshelwoodmechanism.94 This process thus inevitably leads to significantsurface coverage by N atoms. In the plasma-catalytic setup, onthe other hand, surface-adsorbed N atoms react with excitedNH3* molecules and with NH· radicals, thereby forming N2,which can desorb from the surface. In the plasma-catalyticprocess, the reaction thus proceeds through an Eley−Ridealmechanism.94

Lower Activation Barriers and Higher Pre-exponentialFactors. Further, the modification of the reaction pathwaysmay also lead to a change in activation barrier and pre-exponential factor. Nozaki and Okazaki investigated steamreforming of methane and found through Arrhenius-plotanalysis an activation barrier of ∼100 kJ/mol in the reaction-limited regime, for a setup combining both plasma andcatalyst and catalyst alone.95 The pre-exponential factor,however, was found to increase by a factor of 50 in the caseof plasma catalysis. The same observation was made in thediffusion-limited regime, albeit in this case the increase in pre-exponential factor was only a factor 7. The increase in pre-exponential factor can be ascribed to the significantlyincreased dissociative adsorption of vibrationally excited CH4at the catalyst surface compared to vibrationally ground-stateCH4. This conclusion quantifies the contribution of thevibrationally excited plasma species in the plasma-enhancedcatalytic processes, which proceed quite differently comparedto common thermally activated processes.Next to increasing the pre-exponential factor, igniting a

plasma in contact with the catalyst may also lower theactivation barrier. A convincing example was shown fordestruction of toluene on Ag2O/Al2O3.

93 Through Arrhenius-plot analysis, a drop in overall activation energy from 63.2 to49 kJ/mol, i.e., by >20%, was observed. This drop was



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attributed to the generation of surface oxygen radicals. Incontrast, no such change in activation energy was observed inthe case of a MnO2/Al2O3 catalyst. Instead, an increase inpre-exponential factor was found, signifying an increase inactive catalytic centers, which was related to a change inoxidation state of the Mn-oxide, as described above. Note,however, that the use of the Arrhenius equation is limited, inprinciple, to thermal processes, and its application is notstraightforward for plasma processes. Hence, while the synergydemonstrated in these experiments is clear, the quantitativevalues should be treated with care.

3.3. Plasma−Catalyst Interactions and Synergy 2: Effectsof the Catalyst on the Plasma

Some effects of the catalyst on the plasma have been reportedas well: an enhancement of the electric field due to catalystnanofeatures, formation of microdischarges in porous catalysts,and changes in discharge type, all of which are closely related.Probably the most often described effect of the catalyst on

the plasma is the enhancement of the electric field near thecatalyst surface. This enhancement may occur both in the caseof a nanostructured catalyst film and for catalyst-coateddielectric packing in the form of pellets, granulates, or fibers.The field enhancement results from the high local curvatureof the surface, in addition to the possible accumulation ofcharges and polarization effects in the case of dielectricmaterials. While the field enhancement does not directlymodify the chemical properties of the catalyst material, it doesmodify the electron energy distributionwhich in turndetermines the plasma composition and, hence, the relativefluxes of plasma species reaching the catalyst. For instance, Tuet al. demonstrated a clear increase in the high-energy tail ofthe electron energy distribution upon packing their DBD withTiO2 pellets. Especially zeolites and certain ferromagneticmaterials with a high dielectric constant such as BaTiO3 haveoften been demonstrated to be effective materials in plasma-catalytic destruction of VOCs.96−99 Although not specified inthe original works, the strong field enhancement may berelated to the strong nonuniformity of the electric field linesnear the nanopore openings in zeolites and microscopic-scalepolarizability of BaTiO3.Upon partially filling the plasma reactor with packing

material, e.g., dielectric beads or catalyst-coated pellets, thetypical discharge mode is the formation of filamentarymicrodischarges.100 However, when fully packing the reactor,the discharge volume is strongly reduced, leading to amodification of discharge mode from filamentary discharges topredominantly surface discharges,101 which in turn may leadto a decrease in CH4 and CO2 conversion in plasma-catalyticDRM.Moreover, partially or completely filling the discharge

volume will also locally modify the electric field. Thestrongest manifestation of this is seen when using zeolitesor ceramic foams. Indeed, in this case, the electric field insidethe pores is very strong, which modifies the dischargecharacteristics. Hensel et al. demonstrated the effect of thepore sizes on the discharge formation and its characteristics.When pores with a size beyond 15 μm were used, a stabledischarge could be maintained inside the pores.30,102 It ispossible that, below this size, the surface recombination ratesexceed the rates of plasma species generation, thus causing thedischarge to become unstable or even extinguish.

3.4. What Possible Synergies Are Currently Unexplored?

As is clear from the above, the interaction between plasmasand catalyst is complex, and often the net result of combiningboth cannot be attributed to a specific single process. Whereassome factors may prevail, such as the influence of the packingin DBDs on the resulting electric-field distribution and theresulting net effect on the plasma-catalytic process, otherfactors have not been considered in much detail yet. Wedescribe here a number of such factors that may have asignificant effect on the process.

3.4.1. Vibrationally Excited Species. As discussed insection 2.1.2, the Polanyi rules state that translational energyis more efficient than vibrational energy in activating earlyactivation barriers whereas the reverse is true for lateactivation barriers. Vibrational excitation is particularlyimportant in some plasmas, such as microwave dischargesand gliding arcs. In these plasmas, the amount of energy thatis deposited in vibrational excitations can be tuned to someextent. As far as the kinetics of plasma−surface reactions isconcerned, this has two important consequences, as shown inFigure 8.35,36 First, some of the vibrational energy may be

used to decrease the energy barrier to be surmounted, byincreasing the reactant energy by an amount Evib, as shown inFigure 8a. In this case, the barrier for the back reactionremains unaltered. Note that not all vibrational energycontributes to this, as the vibrational coordinate in generaldoes not fully map onto the reaction coordinate. Second,vibrationally excited species may also experience a loweractivation barrier than ground-state species, as ground-statespecies may not have access to the portion of phase spacecontaining the lowest transition barrier. This is shown inFigure 8b, where the amount of lowering of the activationbarrier due to this second effect is denoted by ΔE*. In thiscase, the activation barrier for the back reaction is decreasedas well. The resulting energy barrier due the combination ofboth effects is Ea

v3, which is lower by an amount Evib + ΔE*relative to the energy barrier Ea

v = 0 experienced by ground-state species.Clearly, the interplay between catalyst and vibrationally

excited species is very complex. To the best of our knowledge,only a few studies have focused on this interaction in thecontext of plasma catalysis, and especially in the field ofDRM.95 We expect to see an increased research focus on theeffects associated with vibrationally excited species in plasma

Figure 8. Effect of vibrational excitation of CH4 on the activationbarrier for adsorption on methane. Vibrational excitation maydecrease the effective activation barrier by increasing the energy ofthe reactants by an amount Evib (a) but also by allowing access to anotherwise inaccessible part of phase space, thus lowering theactivation barrier further by an amount ΔE*. Reprinted withpermission from ref 35. Copyright 2004 AAAS.



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catalysis in the near future, and we shall return to this issue insection 5.4.3.4.2. Plasma Photocatalysis. A plasma is generally a

source of both photons and electrons, both of which mayinduce electronic excitations, which in turn may initiatecatalytic surface reactions. In the cases of metal and metaloxide catalysts, irradiation of the catalyst surface by photonsmay enhance the catalytic activity.While the lifetime of electronic excitations at metal surfaces

is very short (in fact, much shorter than the time scale fornuclear motion),103 this is (in part) compensated by the highphoton absorption probability in the near-surface region ofthe metal. The photon absorption leads to photoexcitation ofthe electrons. These electrons then relax through scatteringevents, which in turn leads to a time-dependent electronenergy distribution (see left-hand side of Figure 9). These so-

called hot electrons may attach to empty adsorbate states,thus leading to the formation of an excited adsorbate complex(see right-hand side of Figure 9, showing the transition fromthe ground-state potential to an excited state). If this excitedstate is repulsive, the adsorbate may accumulate sufficientkinetic energy to induce bond breaking while returning to theground state.In the context of plasma catalysis, however, the possibility

for plasma-induced catalytic reactions has been consideredmostly for metal oxides such as TiO2a well-knownphotocatalyst. It has been proposed that intense irradiationof the catalyst by plasma-generated photons may beresponsible for the TiO2 catalytic activity in VOC abate-ment.71,104,105 In this case, a photon energy of minimum 3.2eV is required to excite an electron from the anatase TiO2valence band to the conduction band. In contrast to metals,the recombination of these charge carriers is relatively slow.The electron in the conduction band can reduce an acceptormolecule, while the photoinduced hole in the valence bandcan oxidize a donor molecule. This donor molecule may be,for instance, water or O2, leading to the formation of an OHradical and the superoxide O2

− anion. Both species are highlyreactive and able to oxidize VOCs. However, some reportsindicated that the flux of (plasma-generated) photons in atypical discharge is insufficient to induce an appreciablephotocatalytic effect.80,81,106,107 In contrast, photocatalystactivation was only found to occur under additional UV-irradiation. Therefore, unambiguous proof of plasma-inducedphotocatalysis has not been reported as yet, and it is clear thatsubstantial research is needed in this area.

3.4.3. Collision-Induced Surface Chemistry. Theenergy required to overcome the surface reaction barriertypically comes from the adsorbate and/or adsorbent thermalfluctuations. As discussed above, this energy or part of thisenergy may be delivered through vibrational excitation, or byimpinging photons or electrons. Yet another energy deliverychannel is through the kinetic energy of impinging inertatoms and/or ions. The first observations of this process wereon Ar-impact induced CH4 dissociation on a Ni(111)surface.103,108,109

This process may be resolved in two steps. First, theimpinging atom transfers a fraction of its energy to theadsorbed CH4 molecule. The amount of energy transferreddepends on the position and direction of the impact. Thetransferred energy may then be used to dissociate themolecule in the second step. Thus, the adsorbed moleculeis accelerated toward the surface, which may or may not leadto dissociation, depending on the precise geometry of theadsorbate/adsorbent system. If the molecule does notdissociate, it may desorb from the surface. In fact, it wasfound that the probability of collision-induced desorption issignificantly higher than that of collision-induced dissocia-tion.108

In the context of plasma catalysis, this principle has beenproposed as the mechanism underlying ion-induced networkhealing in plasma-catalytic growth of carbon nanotubes.65 Itwas found that, in a narrow energy window of 15−25 eV,impinging Ar ions transfer sufficient energy to the growingnetwork to reorganize itself, leading to the formation of amore crystalline and less amorphous structure, but they donot transfer enough energy to break up the network anddestroy the structure. Hence, by fine-tuning the surface biasand, hence, the ion kinetic energy, specific processes requiringwell-defined energies may be promoted through thismechanism.

4. CONTRASTING CATALYTIC, PLASMA, ANDPLASMA-CATALYTIC PROCESSING: CNT GROWTHAND CO2 CONVERSION

4.1. Carbon Nanotube Growth

The first case study to illustrate synergy in plasma catalysis isthe growth of one-dimensional (1D) carbon nanostructureson solid surfaces exposed to either a hydrocarbon source gasor a low-temperature nonequilibrium plasma. These examplesinclude single- and multiwalled carbon nanotubes (SWCNTsand MWCNTs, respectively) and carbon nanofibers (CNFs).Although the SWCNT thickness may vary from 0.5 to 10

nm or even more, the most typical thickness is of the order of1−2 nm. MWCNTs and CNFs broadly vary in size, and theirtypical thickness is of the order of tens of nm. The thicknessof these structures is usually limited by the catalystnanoparticles (CNPs) or catalyst-surface features (CSFs),which support nucleation and growth of the nanostructures.110

In the case of SWCNTs, however, there is no uniquecorrelation between the CNP diameter and the tube diameter.Indeed, two distinctive tangential and perpendicular nuclea-tion and growth modes are possible.111 In the tangentialmode the tube diameter is close to the CNP diameter,whereas in the perpendicular mode the tube diameter issmaller than the CNP diameter. Both modes were shown tooccur independently of CNP size.111

Figure 9. Energy diagram for an adsorbate-covered metal surfaceunder the influence of light absorption. EV and EF denote thevacuum level and the Fermi level, respectively. Reprinted withpermission from ref 103. Copyright 2000 Elsevier.



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The growth requires a source of carbon atoms, which canbe delivered to the CNP or CSF surfaces by the chemicalvapor deposition of hydrocarbon precursor gases such asmethane (CH4), acetylene (C2H2), ethylene (C2H4), andothers.112−114 Most often, a hydrogen-based etching carriergas, such as H2 or NH3, is added to remove amorphouscarbon deposit. The precursors need to be decomposed(dissociated) to release carbon atoms that can then nucleateand give rise to the initial nuclei that eventually develop intothe 1D nanostructures. Dissociation of hydrocarbon pre-cursors has a specific energy barrier that needs to beovercome, which depends on the geometrical and electronicproperties of the nanostructure (see section 2.2.2). Apart fromheating the precursor gas flow in the reaction chamber, thecommon means of supplying energy required for thenucleation and growth is by heating the deposition substrate.The exact amount of energy required is determined by thesurface material and topography. Moreover, besides determin-ing the specific mechanisms and activation energy ofhydrocarbon precursor dissociation, they also govern thecapturing and directing of carbon atoms into the sites ofnucleation and incorporation into the developing nanostruc-tures.The pressure in both thermal-catalytic and plasma-based

CNT growth can vary from the sub-Torr range up toatmospheric pressure.63 Traditionally, plasma processingmakes use of reduced pressures, allowing easy plasma ignitionand the formation of a homogeneous discharge. Atmosphericpressure setups, however, do not require expensive vacuumpumps, which is advantageous in an industrial setting.Moreover, ion bombardment at reduced pressure may damagethe growing structure. In contrast, because of the much highercollision rates, ion-induced damage is strongly reduced inatmospheric pressure growth systems. Hence, we envisageatmospheric pressure sources as the current and futuresystems of choice for CNT growth.The outcomes in terms of nanostructure growth are the

results of interaction of the neutral precursor gases in thermalchemical vapor deposition (CVD) or partially ionized gases inplasma-enhanced CVD (PECVD) with the surfaces. They alsodepend on the way the surface texture is formed.The two most common ways to form such features is by

either depositing (or forming otherwise) nanoparticles orsurface texturing. The particles and the textures made ofdifferent materials exhibit very different solubilities of carbonatoms. Generally, nanometer-sized CNPs or CSFs catalyticallyfacilitate precursor gas dissociation and help capture the as-produced carbon atoms to direct them to the nanostructurenucleation sites. These nucleation sites are random and,hence, poorly controllable on smooth surfaces. Importantly,nanostructure nucleation on smooth surfaces may not even bepossible. In this case the CNPs or CSFs are the decisiveenabling factors in 1D nanostructure nucleation and growth.Let us now consider the difference between metal-catalyzed

CVD, PECVD without catalyst, and metal-catalyzed PECVD,focusing specifically on the synergistic plasma-specific effectsthat arise.4.1.1. Thermal Growth with a Catalyst. CNTs are most

often grown by thermal metal-catalyzed CVD. The growthprocess is commonly accepted to follow the vapor−liquid−solid (VLS) mechanism, as shown schematically in Figure10.63

In this mechanism, originally proposed in 1964 for thegrowth of Si-whiskers115 and subsequently applied to thegrowth of carbon nanofibers,116 the gas-phase hydrocarbonprecursor molecules are catalytically decomposed on thesurface of the catalyst nanoparticle. The catalyst is assumed tobe liquid, allowing the carbon to easily diffuse through thebulk of the particle. After supersaturation, carbon starts tosegregate in order to form a bent graphene patch partiallycovering the nanoparticle. By continuous addition of carbonfrom the bulk of the particle, which is delivered through acontinuous influx of hydrocarbon precursor molecules, a solidCNT eventually emerges.The assumption that the catalytic particle is liquid is indeed

justified by the typically used growth temperatures, in theorder of 1000 K, in combination with the Gibbs−Thomsoneffect, as discussed in section 2.2.2. Indeed, often used metalssuch as Fe, Co, and Ni show melting temperatures in theorder of or slightly below that value.55−57 Moreover, atomisticsimulations pointed out that the activation energy for metal−metal and metal−carbon bond switching decreases withincreasing carbon concentration in the nanoparticles. Thisleads, in turn, to an increasing amorphization of thenanoparticles, thus rendering them liquid at an even lowertemperature than would be expected based solely on theGibbs−Thomson effect for the pure metallic nanoparticles.117

The catalyst nanoparticle may be located both at the top orthe bottom of the 1D nanostructure as shown in Figure 11.118

The first case is more common for MWCNTs and CNFswhereas the second case is more relevant to SWCNTs. Theinterface between the CNP and the nanostructure is acceptedas the point of incorporation of carbon atoms into thedeveloping structure. The only significant difference betweenthe CNT and CNF cases is the wall shape resembling straightcoaxial cylindrical walls for CNTs and a bamboo-like structurefor CNFs.

4.1.2. Plasma Growth without a Catalyst. AlthoughCNT growth usually requires a catalyst, both SWCNTs andMWCNTs can also be grown without a catalyst, providedsome other curved seed particle is available allowing for capformation and continued growth.119−121 In this case, the

Figure 10. VLS model applied to CNT growth. (a) Adsorption ofgas-phase hydrocarbon species on the nanocatalyst particle; (b)catalytic decomposition into carbon atoms and dissolution in theliquid bulk; (c) surface carbon segregation with the formation of asolid precipitate; (d) formation of a solid crystalline structure.Reprinted with permission from ref 63. Copyright 2012 AmericanVacuum Society.



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plasma may facilitate the growth process. One possibility is togrow MWCNTs directly on an oxide-free Si surface inmethane-based microwave plasmas without any externalheating, as shown in Figure 12.122

According to the Si/C phase diagram,123 bulk Si has only alimited maximum carbon solubility of 9 × 10−4 at% at 1674 Kdue to the formation of β-SiC precipitates and, hence, doesnot catalyze CNT growth in the way that is common formetal CNPs discussed earlier. In this case, the extent ofcontact of the plasma with the surface plays the key role.122

When the Si surface was smooth, both CVD and PECVDfailed to produce CNTs. When arbitrary small scratches anddots were made on the Si surface, only direct plasma contactwith the surface produced the nanotubes. Importantly, boththermal CVD and remote plasmas failed to produce theMWCNTs.Surface microscopy revealed that microscopic Si nano-

features with tip sizes of the order of 10 nm were produced

upon scratching. As mentioned in section 2.3.2, the plasmacan provide heating necessary for the nucleation and growthof carbon nanotubes or nanofibers. The surface temperatureat the discharge power that produced the nanotubes matchedthe melting temperature of a Si nanoparticle of a ∼10 nmsize.122 Once molten, Si easily forms a carbide phase whenexposed to a flux of carbon atoms. SiC particles are known tocatalyze CNT growth,124,125 and this was a likely reason forthe formation of MWCNTs only on Si NFs and only upondirect contact with the plasma.This manifests a synergistic effect: the process only works

when both the Si nanofeatures and the plasma are present.Exposing a smooth Si surface to a plasma or Si nanofeaturesto a neutral gas of the equivalent CVD process both did notwork.122

4.1.3. Plasma-Catalytic Growth. Plasma-catalytic growthis rather common for SWCNTs, MWCNTs, and CNFs, andmany works reported on the beneficial effects of combiningthe catalyst with the plasma.61−63,126 Compared to thermalgrowth, a large variety of active species may contribute to thegrowth process, including atoms, molecules, excited meta-stable species and radicals, ions and electrons, and photons, asshown in Figure 13. Typically, electric fields are also present,further modifying the growth conditions relative to thermalmetal-catalyzed growth.61,63

In a plasma-catalytic process, the nonequilibrium reactiveplasma chemistry leads to the effective precursor conversionin the gas phase, as described in section 2.3.1, and also to anaccelerated carbon species production at the catalyst surface.Moreover, these carbon species can be delivered to thegrowth surface faster in the plasma, for example, through theion fluxes. The contributions of these fluxes can be verysignificant, even in comparison to the contributions of neutralradical species due to their typically high sticking coefficients.This will lead to larger amounts of carbon atoms on thesurface or dissolved within CNPs, which may on the onehand lead to lower diffusion barriers and higher bondswitching rates.117 On the other hand, CNP supersaturationmay occur faster, thereby shortening the lead time before thenucleation (incubation time).127 However, increased C-fluxesmay also cause CNP poisoning or even completeencapsulation of the catalyst by amorphous carbon.

Figure 11. Two common locations of metal catalyst nanoparticles inthe growth of MWCNT (left) and SWCNT (right) nanostructures.Reprinted with permission from ref 118. Copyright 2008 John Wileyand Sons.

Figure 12. Schematic of the microwave plasma CVD system (a);melting of the topmost part of the Si nanofeature (NF) andformation of a solid SiC hemispherical tip (b); elementary surfaceprocesses involved in the heating of the Si NF and generation of Catoms (c); formation of the graphene cap (d); and growth of thevertically aligned nanotubes (e). The important surface processessketched in (c) are ion-induced dissociation (IID), ion decom-position (ID), nitrogen ion heating (NH), and thermal dissociationof hydrocarbon radicals (TD). Reproduced with permission from ref122. Copyright 2013 John Wiley and Sons.

Figure 13. Factors affecting the growth of CNTs in a PECVDprocess. Electromagnetic fields are not shown. Reprinted withpermission from ref 63. Copyright 2012 American Vacuum Society.



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Ion bombardment may play an important role in plasma-enhanced catalytic CNT growth. During the SWCNT capnucleation in particular, ion bombardment leads to the top-down energy transfer from the plasma to the C-network. Thismakes it possible first to improve the structural quality of theC-network,65,66 as shown in Figure 14. Moreover, the ion

bombardment may also provide the additional “bendingenergy” (see Figure 15) to lift up the initial as-nucleated

graphene monolayer off the CNP surface to form a SWCNTcap.128 Ion bombardment may change the growth mechanism,as has recently been shown by atomic-scale simulations.77,78

However, ion bombardment may be detrimental for CNTgrowth, leading to etching and sputtering.67−70,129 Adiscussion of the beneficial and detrimental effects of ionbombardment during CNT growth can be found elsewhere.130

The local temperature of the CNPs can also increase due tothe effects of ions and other plasma species (see section2.2.2). In the case of plasma-catalytic CNT growth, this effectmay lead to nanostructure nucleation using reduced substrateheating.131 In this way, the nanostructure growth rates can besubstantially increased.132

The importance of both ion bombardment and plasma-induced heating is highlighted by numerical simulations onCNF growth. Figure 16 shows a range of reactions that takeplace on the surface of a Ni CNP located on top of a CNF,exposed to low-temperature nonequilibrium plasma.Most of these processes and factors (e.g., adsorption,

desorption, activation energies, etc.) are similar for both CVDand PECVD. However, the presence of ion-induced reactionsand localized plasma heating makes a major differencebetween these two processes, including the rates of similarprocesses. Indeed, the relative importance of carbon atoms

diffusing over the surface or through the bulk of the CNP(termed surface (SD) and bulk (BD) diffusion in Figure 16,respectively) is very different between CVD and PECVD andin different temperature ranges.Both experiments133 and numerical modeling134 of CNF

growth in the plasma suggest that surface diffusion dominatesin low-temperature growth of CNFs in the plasma whereasbulk diffusion is the main mechanism in thermal CVD ofCNFs at higher temperatures. This is shown in Figure 17.Indeed, Figure 17 shows the growth rates, Hs and Hv,corresponding to the surface diffusion and bulk diffusionchannels, respectively, taken separately. In Figure 17b, theblue curve corresponding to the surface diffusion channel fitsto the experimental curve.135 This fit is particularly clear inthe lower temperature range where the purely thermal CVDgrowth is not even possible (1000/Ts > 1.4, where Ts is thesurface temperature). On the other hand, the bulk diffusionmechanism quantifies the thermal CVD growth in the highertemperature range where 1000/Ts < 1.4. We find thisnumerical result to be in good quantitative agreement withthe available experimental data.136

Therefore, one of the synergistic effects of the plasma andthe Ni CNPs where purely thermal CVD growth is inredirecting carbon atoms to the nanofiber incorporation sitesthrough the catalyst surface at lower temperatures when theirdiffusion through the bulk becomes ineffective. This leads tothe growth of CNFs in the temperature range below 714 K(yet above 400 K) where purely thermal CVD growth isimpossible.When only plasmas are used without catalyst nanoparticles,

1D nanostructures do not nucleate on smooth surfaces withlow carbon solubility, as described above. When CNPs areexposed to the plasma, the growth rates are usually higher atthe same external heating temperatures compared to thermalCVD using the same precursor gas under the sameconditions. Alternatively, the same growth rates can beachieved in PECVD at much lower external heatingtemperatures than in CVD.We emphasize that in several cases reported in the

literature the external heating may even become completelyredundant as was the case for the MWCNT growth described

Figure 14. Ion-induced improvement of structural quality of aSWCNT cap on a Ni CNP in the ion energy range of 15−25 eV.Reprinted with permission from ref 65. Copyright 2013 AmericanPhysical Society.

Figure 15. Bending energy ΔE as a function of the substrate holdertemperature TH for PECVD and thermal CVD processes. Reprintedwith permission from ref 128. Copyright 2011 American ChemicalSociety.

Figure 16. Range of plasma-affected elementary processes on thesurface and bulk of a Ni CNP in PECVD of carbon nanofibers. Thefollowing processes are sketched: AD = adsorption of C2H2, DS =desorption of C2H2, DIS = dissociation, EV = evaporation, SD =surface diffusion, INC = incorporation into a graphene sheet, BD =bulk diffusion, ADH = adsorption of H atoms, DSH = desorption ofH atoms, LAP = loss of adsorbed particles at interaction with atomichydrogen, IID = ion-induced dissociation of C2H2, and ID = iondecomposition. Reprinted with permission from ref 134. Copyright2007 AIP Publishing LLC.



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in section 4.1.2. This indicates the constructive, synergisticinteraction between the plasma and the CNPs, which leads tothe reduced amounts of external heating required to nucleateand grow the nanostructures and to increase the nanostruc-ture growth rates.Another clearly synergistic effect of the plasma and CNPs is

vertical alignment along the direction of the applied electricfield in the plasma sheath near the substrate surface. Whereasthermal CVD processes usually produce tangled, twistedCNTs with random orientations and growth directions unlessdense crowding effects guide CNTs growth vertically, PECVDproduces vertically aligned, free-standing single-walledCNTs.137−140

This common observation clearly shows the synergisticeffect defined in section 3. Indeed, just using plasmas and verysmooth surfaces usually produces (e.g., amorphous) carbonfilms rather than localized CNTs. Therefore, plasma requiressurface features or catalyst to produce CNTs. When catalystparticles are used in conjunction with thermally dissociatedflow of gas precursor (thermal CVD), the nucleation/growthtemperatures are notably higher (at least a couple of hundreddegrees Celsius) than when the plasma is used. In this casethe nanotubes are thicker, the chirality distribution is broader,and the orientation is random. When both plasma and catalystare used, the growth temperatures are notably lower,alignment is vertical, the tubes are thinner, and the chiralitydistribution can also be narrower.141,142 This observation mayin part be attributed to the combination of the electric fieldand ion bombardment, allowing for nucleation at lowerexternal heating, with smaller radii, and with predeterminedvertical orientation.It is thus quite clear that the various plasma-specific factors

may enhance the growth, also in the case of catalyst materialswith low solubility of carbon atoms, such as Au. This suggeststhat if plasma enables growth of 1D (and 2D as we will see insection 5.1) nanostructures at low temperatures and surfacediffusion is the dominant mechanism, then there is no realneed to use catalysts with high carbon solubility in this low-temperature range. Because plasmas also increase the rates ofgeneration and delivery of precursor species to the growthsurfaces, the nanotubes can be grown very quickly. A strikingexample is the possibility to grow short single-wallednanotubes using Au catalyst nanoparticles, as demonstratedby both experiments and simulations.128,141,143 Specifically,

SWCNTs (an example is shown in Figure 18) were detectedeven after only 2−15 s of the plasma-discharge operation.

Thin SWCNTs with a relatively narrow chirality distributionwere produced at temperatures 580−640 °C, inaccessible bythermal CVD for the given catalyst nanoparticle size range.For SWCNTs with (8,6) chirality, the difference between thenucleation temperatures in thermal and plasma CVD is ∼200degrees.In addition to the previously mentioned vertical-alignment

effect, the electric field generated in the plasma sheath (seesection 2.3.1) also plays a major role at the nucleation stage ofCNTs. As shown by atomic-scale numerical simulations,138

the electric field should be sufficiently strong to play asignificant role in the nanotube growth. First, these electricfields ensure the vertical growth direction and SWCNTalignment, in agreement with experimental results.137 Second,the graphitic network shows a better ordering compared tothe cases when there is no electric field or when the electricfield is weak. Third, similar to the CNF and MWCNT cases(as well as other nanostructures such as Si nanowires), thegrowth rates of SWCNTs are increased by the application ofthe electric field.The electric field increases the degree of directionality of

carbon atom movement along the CNP surfaces as opposedto the completely random motions in all directions withoutthe electric field. Because the direction of the electric field inthe plasma is normal to the surface, it guides polarized (seeexplanation below) carbon atoms upward, toward the top of

Figure 17. (a) Arrhenius plots for CNF growth rates on Ni, Co, and Fe catalysts in NH3-diluted C2H2 in thermal (dotted line) and plasma-enhanced CVD. Reprinted with permission from ref 133. Copyright 2005 American Physical Society. (b, c) Growth rates Hs, Hv, and Ht = Hs +Hv as functions of the surface temperature for PECVD (b) and CVD (c). The triangles and circles represent the experimental points.135,136

Reprinted with permission from ref 134. Copyright 2007 AIP Publishing LLC.

Figure 18. Single-walled carbon nanotubes grown using Au catalystnanoparticles. Plasma-assisted CVD enables very fast growth atsignificantly reduced temperatures. Reprinted with permission fromref 143. Copyright 2010 American Chemical Society.



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the CNPs, eventually giving rise to the nucleation of SWCNTcaps at the summit of the catalyst nanoparticle. This is verydifferent from common thermal CVD cases where nucleationof CNT caps occurs randomly, leading to disordered,misaligned growth, which is often referred to as a“spaghetti-type” growth.144,145

Importantly, this phenomenon arises through the combinedeffects of the plasma and the catalyst nanoparticle. WithoutCNP, the carbon atoms are charge-neutral. However, directcontact with a Ni nanoparticle during the SWCNT capnucleation stage leads to charge transfer in the C−Ni system,as shown in Figure 19.138 Carbon atoms acquire a smallnegative charge while Ni atoms become slightly positivelycharged. The plasma electric field can then act on thispolarization to induce directed growth. Note that polarizationof carbon atoms on noncatalytic surfaces is weaker or absentcompared to the case of the Ni−C system. This demonstratessynergistic effects of the plasma-produced electric field and Nicatalyst nanoparticles, which then lead to better growthdirectionality with higher rates and improved quality of thegraphitic network.In combination with other plasma effects (higher super-

saturation of carbon within CNPs and higher local temper-atures on their surfaces, in addition to nonthermal surfaceactivation), the synergistic effects of the CNPs and electricfield are further amplified by allowing for the nucleation ofthinner nanotubes at lower temperatures of external heatingwith predetermined base positions (at the CNP summits),and also possibly with narrower chirality distributions, inaddition to the common vertically oriented growth. Thisshows clear synergistic effects in plasma-catalytic nucleationand growth of CNTs. We emphasize that quite similarsynergistic effects may be applicable to other related 1Dnanostructures. This opens an interesting opportunity forfuture research.4.2. CO2 Conversion

The second case study to illustrate the synergy of plasmacatalysis is for gas conversion. This includes air pollutioncontrol (e.g., destruction of volatile organic compounds,VOCs), CO2 conversion into value-added chemicals, hydro-carbon reforming, ammonia production, etc. Several exampleswill be given in section 6. Here, we describe greenhouse gas(i.e., CO2 and CH4) conversion into value-added chemicals ornew fuels, such as syngas (a mixture of CO and H2),methanol, formaldehyde, and other hydrocarbons or oxygen-ates. This process is gaining increasing interest worldwide,because of the global climate change and the increasingenergy consumption. Indeed, these greenhouse gases couldconstitute an alternative for petroleum, which will become lessavailable and therefore more expensive in the future. Theconversion of these greenhouse gases is therefore considered

as one of the main challenges for the 21st century.146,147

Moreover, as this process aims to convert waste (i.e.,greenhouse gases) into a new feedstock (raw materials forthe chemical industry), it can be considered to fit the “cradle-to-cradle” concept.148 However, the conversion process ofCO2 and CH4 is not straightforward, because both moleculesare thermodynamically stable, and their dissociation requiressignificant amounts of energy. Here, we will critically examinethe state of the art and current challenges for the conversionof CO2 (and CH4), based on thermal-catalytic processing,plasma conversion without catalyst, and plasma catalysis.

4.2.1. Thermal-Catalytic Conversion. Splitting CO2 intoCO and O2 is an endothermic process with a reactionenthalpy of 283 kJ/mol or 2.9 eV per molecule, at 298 K:149

→ +CO CO 1/2O2 2 (2)

The equilibrium of this reaction lies strongly to the left,149

such that the equilibrium yield of CO and O2 varies fromabout (or less than) 1% at T < 2000 K up to ∼60% attemperatures between 3000 and 3500 K.150 The equilibriumconstant as a function of temperature can be found in thework of Wagman et al.149

Active removal of one (or both) products increases theconversion, following Le Chatelier’s principle. This may beaccomplished by the use of zirconia membranes, asdemonstrated by Nigara and Cales151 and Itoh et al.152 Byusing a calcia-stabilized zirconia membrane and CO as sweepgas, Nigara and Cales reached conversions of 21.5% at 1954 Kin the most reducing conditions, to be compared to aconversion of CO2 of no more than 1.2% at this temperaturein thermal splitting. The overall conversion, however, wasmuch lower and in line with the overall thermodynamicequilibrium conversion, due to permeation of O2 through themembrane reacting with the CO sweep gas, thereby formingCO2.

150,151

Approximately 92 kJ/mol is needed to heat 1 mol of CO2from 300 to 2000 K, while the reaction enthalpy is equal to245 kJ/mol at 2000 K. On the basis of a conversion of 1.2%,the energy cost for total conversion is ∼7.9 MJ/mol, yieldingan energy efficiency of 3.6% with respect to the reactionenthalpy of 283 kJ/mol at 300 K. To heat 1 mol of CO2 to3500 K, on the other hand, ∼184 kJ/mol is needed, while atthis temperature the reaction enthalpy is equal to 206 kJ/mol.On the basis of a conversion of 60%, the energy cost of totalconversion is then only ∼513 kJ/mol, yielding an energyefficiency of 55% with respect to the reaction enthalpy of 283kJ/mol at 300 K.Hence, it is clear that thermal CO2 splitting is

thermodynamically and energetically only favorable at veryhigh temperatures,149,150 while alternative approaches at lowertemperatures have not yet realized comparable conversions

Figure 19. (a) Charge transfer in the nucleation of SWCNT cap on a Ni CNP; (b) effect of the electric field on the continued CNT growth,leading to vertical alignment. (a) Reproduced with permission from ref 138. Copyright 2012 American Chemical Society.



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and energy efficiencies.150−152 Because thermal-catalytic CO2splitting is very energy consuming, the only practical way ofthermal CO2 conversion is dry reforming of methane, i.e., thesimultaneous conversion of CO2 with CH4, yielding theproduction of syngas:

+ → +CH (g) CO (g) 2CO(g) 2H (g)4 2 2 (3)

Following Le Chatelier’s principle, increasing the pressureof the reactants will drive the reaction backward, and hencethe reaction is commonly carried out at atmospheric pressure.Just like direct CO2 splitting, this is also an endothermicreaction, with a standard reaction enthalpy of 247.3 kJ/mol or2.56 eV per converted molecule. Therefore, it needs to becarried out at high temperatures (600−900 °C), by means ofa catalyst, typically containing Ni, Co, precious metals, orMo2C as the active phase. Substantial research efforts aredevoted to the search of the optimum catalyst materials,153,154

as the process suffers from significant carbon deposition at thecatalyst material, giving rise to catalyst poisoning.155 On theindustrial scale, the process is most efficient at 700 °C,reaching thermodynamic equilibrium conversions of CH4 andCO2 of 72% and 82%, respectively. As the reaction between 1kmol of CH4 and 1 kmol of CO2 at 700 °C requires 260 MJenergy, and at least an additional 70 MJ is required to bringthe gas flow to this temperature, at least about 330 MJ energyis required for this process, resulting in an energy input of atleast ca. 3.42 eV per molecule and a corresponding maximumtheoretical energy efficiency of 58%. It is crucial to realize thatthis is the theoretical energy efficiency. We emphasize that, toestimate the overall energy efficiency, one also has to accountfor the thermal efficiency of the heaters, which depends onthe type of heater, the type of fuel, and the use of heat-recovery systems.4.2.2. Plasma Conversion without a Catalyst. In recent

years, there has been an increasing interest in the use ofplasma technology for CO2 conversion. Experiments arecarried out in several types of plasmas. The most commontypes reported in the literature are DBDs,156−158 micro-wave,159,160 and gliding arc discharges.161,162 The first type is anonequilibrium or cold plasma, where the gas is more or lessat room temperature, and the electrons are heated totemperatures of 2−3 eV by the strong electric field in theplasma. The microwave and gliding arc plasmas areconsidered to represent the so-called warm plasmas, anintermediate case between thermal and nonthermal (cold)plasmas. The gas can reach temperatures up to 1000 K andmore, and the electron temperature is typically ∼1 eV. Thistemperature is ideal for populating the vibrational levels ofCO2, and because the CO2 splitting is most energy-efficientwhen it proceeds through the vibrational levels,5,163 warmplasmas are more advantageous in terms of energy efficiencyof the process. Indeed, the highest energy efficiency for pureCO2 splitting reported for a microwave reactor is 90%.5

However, we emphasize that this result was obtained atreduced pressure (∼100−200 Torr), and increasing theprocess pressure to atmospheric pressure, desirable forindustrial applications, dramatically reduces the energyefficiency. The highest energy efficiency for CO2 splitting ina gliding arc plasma, operating at atmospheric pressure, isreported to be 43%, at a conversion of 18%.161 For DRM, aconversion of 8−16% was reported, with a correspondingenergy efficiency of 60%.162

While the energy efficiency reached with DBDs isconsiderably lower, i.e., in the order of 4−8% for both pureCO2 splitting and for DRM,164,165 a DBD reactor has a verysimple design, which is beneficial for upscaling, and operatesat atmospheric pressure, making it suitable for practicalapplications. It can also easily be combined with a (catalytic)packing, as will be elaborated in the next section, which isimportant for the selective production of targeted compounds.The important point, which to some extent has been

overlooked in the literature, is the substantial lack ofselectivity when plasmas are used. Indeed, the reactive species,created by the electrons, easily recombine, and a large numberof different products can be formed. In pure CO2 splitting,this is not an issue, as CO and O2 are the maincomponents.165 However, in DRM, typical products formedare syngas, higher hydrocarbons (C2H6, C2H4, C2H2, C3H8,...), as well as oxygenates, such as methanol, ethanol,formaldehyde, acetaldehyde, and carboxylic acids.164,166

Combination with catalysis will then be highly desirable, ifsome specific compounds are targeted.

4.2.3. Plasma-Catalytic Conversion. The selectivity andenergy efficiency of plasma-based CO2 conversion can beimproved by combining the plasma with a catalyst. Plasma-catalytic CO2 conversion usually takes place in a DBD reactor,which typically operates at atmospheric pressure. It isimportant to distinguish between the physical and chemicaleffects of introducing a catalyst in the plasma. In simple terms,the chemical effects are the basis for the improved selectivitytoward the targeted products, while the physical effects aremainly responsible for the better energy efficiency. In the caseof pure CO2 splitting, mainly CO and O2 are formed, so theprimary added value of the catalyst is to increase the energyefficiency, although the conversion can also be improved bychemical effects such as enhanced dissociative chemisorptiondue to catalyst acid/basic sites. In the case of DRM, or otherreactions involving CO2 and a coreactant (e.g., H2O, H2), theselectivity toward targeted products can be tuned (see below).We will first elaborate on the physical effects, before showingsome examples of the chemical effects, although it needs to berealized that both effects are interrelated and cannot always beseparated.The local electric field in the plasma can be enhanced due

to sharp edges of the nanostructured catalyst (see section3.3), or simply at the contact points of the catalyst pellets. Itshould be stressed that this effect also occurs whenintroducing a simple dielectric packing in the plasma, becauseof polarization of the dielectric beads. This enhanced electricfield, for the same applied power, yields higher electronenergies, increasing the ionization and dissocation efficiency ofthe gas molecules, leading to more energy-efficient conversion.The packing or catalyst loading in the reactor is of prime

importance in plasma catalysis for CO2 conversion. Forinstance, DRM was investigated in a single-stage reactorsystem comprising a coaxial dielectric barrier discharge (DBD)reactor combined with Ni/γ-Al2O3 catalysts as a function ofthe packing method.100 Three packing methods wereemployed: full packing, partial packing (axial), and partialpacking (radial). Unlike the full packing method where thegas discharge is limited due to the suppression of thefilamentary microdischarges as a result of low dischargevolume, the partial packing method (radial or axial) shows astrong filamentary discharge due to the large void fraction inthe discharge gap. This improves the physicochemical



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interactions between the plasma and the catalyst, leading tohigh methane conversion (56.4%) and significant hydrogenyield (17.5%).The use of ceramic foams may yield higher conversion

efficiencies, as demonstrated, e.g., by Kraus et al. in a DBDused for DRM.167 This conversion improvement wasexplained by the higher electron energies, arising from thesmaller discharge volumes in the pores of the ceramic foams.Tu and colleagues demonstrated a change in the dischargebehavior (i.e., less filaments and more surface discharges) inthe case of a TiO2 packing (see section 3.3).101 It was notedthat this change affects the electron energy distributionfunction (EEDF), yielding more electrons in the high-energytail. Similar results were also obtained for nonconductive(Al2O3, zeolite 3A, NiO/Al2O3) and conductive (reduced Ni/Al2O3) packings. However, strong filaments were stillobserved for porous quartz wool or small catalyst flakes,where the effect is quite similar to nonpacked DBD,presumably due to the high porosity of the material. Thus,one can conclude that the relative contributions of filamentaryversus surface discharges depends on the particle size, shape,packing location, and, hence, the void fraction.101 The samegroup reported a higher CH4 conversion and H2 yield forquartz wool in the case of DRM, due to changes in thephysical properties (i.e., more intense filaments), whereas inthe case of γ-Al2O3 and zeolite 3A, a lower discharge intensitywas obtained, yielding lower CH4 and CO2 conversions.168

Importantly, these results indicate that a catalyst does notalways lead to better performance of the plasma conversion.Nevertheless, the zeolite 3A catalyst still yielded a betterselectivity toward H2 and light hydrocarbons, while theformation of liquid hydrocarbons was inhibited, due to theshape-selectivity determined by the zeolite pore diameter.Dielectric materials such as glass and ferroelectric materials

such as BaTiO3 may induce a shift in discharge mode.Recently, Mei et al. demonstrated that, in plasma-assistedconversion of CO2, the typical filamentary discharge changes

into a combination of filamentary discharge and surfacedischarge, upon adding the packing. The added packing inturn gives rise to an enhanced average electric field and meanelectron energy due to polarization effects (i.e., by a factor 2for BaTiO3), resulting in a higher CO2 conversion, CO yield,and energy efficiency.168 It is obvious that the plasma ismodified significantly by the catalyst. However, it presentlyremains unclear how the modified plasma properties affect thecatalytic process.Even when the discharge mode is not altered by the

packing, the packing material still has a profound impact onthe overall plasma characteristics and on the CO2decomposition. Yu et al. studied how different packings (i.e.,silica gel, quartz, γ-Al2O3, α-Al2O3, and CaTiO3), withdifferent dielectric constants, pore sizes, and therefore BETsurface areas,169 affect the EEDF in a packed-bed DBD forCO2 conversion (see section 2.3.1). The CO2 decompositionin the case of quartz was higher than that for silica gel,although both materials are chemically inert and have thesame dielectric constant. This different performance could beattributed to the different pellet morphology, i.e., the quartzpellets had rigid edges whereas the silica gel pellets werespherical. The sharp edges lead to an electric fieldenhancement near the contact points and hence to morehigh-energy electrons. Besides the physical properties, theacid−base properties of the packing material also werereported to affect the reaction, through chemisorption ofCO2 on the basic sites. Indeed, CO2 will preferentiallychemisorb on γ-Al2O3, which is an amphoteric oxide, and willalmost not adsorb on α-Al2O3, which is chemically inert.169

Whereas the physical effects of introducing a catalyst in theplasma seem to be quite straightfoward, i.e., change indischarge behavior, local electric field enhancement, effect onthe EEDF, and yielding higher energy efficiencies, thechemical effects of plasma catalysis are clearly less understood.One of the reasons is that the physical and chemical effectsare often correlated, as the catalysts are mostly introduced in

Figure 20. Selectivity for the formation of formic acid (C1), acetic acid (C2), propionic acid (C3), and butyric + 2-methylpropionic acid (C4), inthe case of DRM in a DBD plasma, using nickel, stainless steel, or copper electrodes. Reproduced with permission from ref 170. Copyright 2014John Wiley and Sons.



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a packed bed, so it is not easy to distinguish between the twoeffects.An enhanced selectivity toward desired products can in part

be obtained by judiciously selecting the catalyst material.Scapinello et al. demonstrated an improved selectivity towardthe production of carboxylic acids in the case of DRM, whenusing copper or nickel electrodes instead of stainless steel.170

This effect may be attributed to a pure chemical catalyticeffect of these metals, as no packing was involved. This isillustrated in Figure 20 for the formation of formic acid (C1),acetic acid (C2), propionic acid (C3), and butyric + 2-methylpropionic acid (C4). The effect is most pronounced forformic acid, where the selectivity in the case of nickel is fourtimes higher than when using stainless steel. These resultstaken together indicate that, besides gas-phase reactions,surface reactions, and more specifically hydrogenation ofchemisorbed CO2, play a key role in the synthesis of thesecarboxylic acids.Another reason why the chemical effects of plasma catalysis

for CO2 conversion are not yet fully understood is that a widevariety of catalyst materials are being explored, includingmetals (Au, Pd, Pt, Rh, Cu),171 zeolites (NaY, NaOH-treatedY, HY, Na, NaX, Na-ZSM-5, and Linde-type 5A zeolite),23,172

La2O3/γ-Al2O3, CeO2/γ-Al2O3,173 n-type oxide semiconduc-

tors (ZnO, CuO), and Al2O3 foams with Ni, Rh, or Cacatalysts.174 Commercial Ni/Al2O3 catalysts, which are alsoused in thermal catalysis, are probably the most popularcatalysts for DRM, but they are not necessarily the mostsuitable ones under plasma conditions.These discussions suggest that there is still a lack of insight

into which catalysts should perform best in a plasmaenvironment, and more systematic studies on both thephysical effects (of the structural packing and the supportmaterials, including, e.g., their dielectric constant, acid/baseproperties, and porosity) and the chemical effects of thecatalyst material (including type and coordination of theactive element) are highly needed.A highly important synergistic effect of plasma catalysis is

promotion of catalyst activity at reduced temperatures and,hence, a significant reduction in the energy cost for activatingthe catalyst. Wang et al.,175,176 for instance, illustrated suchsynergy for the single-stage plasma catalysis of DRM with Ni/Al2O3 catalyst but did not observe this synergy in two-stageplasma catalysis or when the catalyst was only placed at theend of the plasma zone. Moreover, the synergy was a bit morepronounced in a fluidized bed than a packed bed, because thefluidized bed reactor favors the interaction between theplasma and the catalyst material. Typical synergistic effectfactors of 1.25−1.5 were obtained, i.e., the conversion in theplasma-catalysis system was up to 50% higher than the sum ofthe conversions in the pure plasma and thermal-catalysis cases.A maximum in the synergistic effect factor was obtained at∼700 K, because at higher temperatures thermal catalysisbecomes gradually equally efficient. It was thus concluded thatthe plasma promotes the catalyst activity at lower temper-ature. We stress that the above conclusion is very similar tothe case of plasma-based SWCNT nucleation and growth onmetal catalyst nanoparticles, as discussed in section 4.1,indicating the closely related nature of these catalyticprocesses.Similar observations as earlier mentioned were also made

by Eliasson et al.177 Figure 21 shows the methanol yield as afunction of temperature in the range from 50 to 250 °C at a

pressure of 8 bar for the hydrogenation of CO2 to methanolin the case of pure thermal catalysis, plasma (DBD) only, andplasma catalysis, using CuO/ZnO/Al2O3 catalytic pellets.Note that the high pressure was needed to favor methanolformation against methanation, which is the main competitivereaction. The methanol yield was only ∼0.1% in the plasma-only case, as well as in the catalysis case at low temperature(∼100 °C). A maximum methanol yield of 2% was obtainedin the case of thermal catalysis at a temperature of 220 °C. Inthe case of plasma catalysis, a maximum methanol yield of 1%was reached at a temperature of 100 °C. This means that theplasma can shift the region of maximum catalyst activity tolower temperatures by >100 °C, which corresponds to asignificant reduction in the energy cost for activating thecatalyst.Very convincing results on the plasma/catalyst synergy in

the case of DRM were presented by Zhang et al., usingdifferent Cu−Ni/γ-Al2O3 catalysts (see Figure 22).178 TheCH4 conversion in the case of plasma-only and thermalcatalysis was 13% and 10%, respectively, whereas themaximum conversion in the case of plasma catalysis was69%. Similarly, the CO2 conversions in the cases of plasma,

Figure 21. Methanol yield as a function of catalyst/wall temperature,in the case of plasma (DBD discharge) only, catalyst only, andplasma catalysis, at a pressure of 8 bar, a plasma power of 500 W (nopower in the case of catalyst only), a flow rate of 0.5 L/min, and aH2/CO2 gas mixing ratio of 3:1. Reprinted with permission from ref177. Copyright 1998 American Chemical Society.

Figure 22. Maximum CH4 and CO2 conversion in the case ofplasma-only, thermal catalysis, and plasma catalysis, clearly illustratingthe synergy of plasma catalysis, in the case of DRM. Reprinted withpermission from ref 178. Copyright 2010 Elsevier.



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thermal catalysis, and plasma catalysis amounted to 2.5%,13%, and 75%, respectively. Furthermore, the selectivitiestoward H2 and CO were much better in the plasma-catalysiscase. A reaction mechanism was put forward, based on theadsorption of reactive plasma species (CHx radicals, O and Hatoms) on the catalyst surface, followed by recombination ofthe adsorbed species. Furthermore, it was suggested that theplasma will heat the catalyst surface and, therefore, enhancethe desorption of the surface species. The proposedmechanisms align well with the discussions of relevantprocesses in the previous sections of this review.Note, however, that synergy in plasma catalysis is not

always observed for CO2 conversion or DRM. For instance,Tu et al.179 showed a conversion of CH4 and CO2 with a Ni/Al2O3 catalyst slightly lower than that without a packing.Similar trends were observed by Sentek et al.180 for theconversion of CH4 with a Pd/Al2O3 catalyst. The changes inthe product selectivities, on the other hand, were considerable,pointing out at least some chemical effects of the catalyst.In general, the enhanced performance of plasma catalysis

can in part be attributed to (vibrational) excitation of CO2(and the coreactants CH4 and H2) in the plasma, whichenables easier dissociation at low temperature on the catalyst.This was also suggested by Amouroux et al.,174 who claimedthat another effect is the expelling of water from the catalystsurface by supplying electrons from the plasma (so-calledelectropolarization). This gives a higher methanol yield inplasma catalysis. Indeed, the plasma electrons affect thecatalyst properties (chemical composition or catalyticstructure). Furthermore, it was stated that short-lived plasmaspecies (e.g., excited O atoms) are formed inside the catalystpores. Thus, a catalyst with high dielectric constant should bedesirable, as this gives more pronounced polarization of thepellets and thus higher electric fields, favoring the dissociationof species inside the catalyst. It was indeed demonstrated thatCaSrTi oxides, which are characterized by high permittivities,give much better CO2 conversion than Al2O3 and SiO2.

174

These conclusions are consistent with our discussion of theelectric field and polarization effects of zeolite and BaTiO3catalysts in section 4.2.3.It is worth mentioning that all these examples are based on

a DBD plasma. Up to now, there are indeed only a limitednumber of efforts for combining a catalyst with a micro-wave181 or gliding arc plasma,182−184 with the latter examplebeing for CH4 reforming. The reason might be that it is lessstraightforward to introduce a catalyst material in theseplasma types, to ensure enough contact time between gas andcatalyst, and to avoid too much unused volume, as well as toguarantee that the catalyst remains stable under highertemperatures. It is our opinion that catalysis with warm

plasmas could be very promising, as the catalyst surface canbe activated by the somewhat elevated temperature, character-istic for warm plasmas (see earlier), possibly leading to extrasynergies.

5. OTHER CURRENT AND FUTURE APPLICATIONS OFNANOCATALYST-BASED PLASMA CATALYSIS

Besides plasma-enhanced nanoparticle catalyzed CNT growthand CO2 conversion, a variety of other processes also showsynergistic effects when the plasma and catalytic effects arecombined. Below we critically discuss nonexhaustive examplesof some of the most interesting effects.

5.1. Catalytic Synthesis of Graphene and RelatedNanostructures

In this section we again consider low and high solubilities ofcarbon atoms in catalyst materials, similar to the CNT growthof section 4.1. We will also consider an interesting case whenplasma-assisted dissolution and diffusion through a nano-catalyst helps to form graphene films on silica surfaces.The first example shows the possibility to grow thin

graphene nanoribbons under reduced pressure using nickelnanobars as growth catalysts, connected to transistorterminals.185 The thickness of the Ni nanobars was in the∼23−105 nm range, height 35−85 nm, and length 200 nm−5μm (see Figure 23). RF plasmas of methane and hydrogenmixtures were used during the rapid (5−60 s) plasmaexposure of preheated Ni nanobars. During this exposure theNi nanobars only melted partially and plasma-producedspecies were dissolved in Ni. During rapid cooling, graphenenanoribbons nucleated and eventually covered the nanobars asshown in Figure 23. The Ni nanobars nearly completelyevaporated, leaving graphene nanoribbons connecting the Niterminals. For sufficiently high Ni nanobars (e.g., >60 nm),interesting graphene nanobridge structures formed while theNi beneath them evaporated.Rapid heating, delivery of species to Ni nanobars, and then

rapid cooling appear to be critical. The nanobars themselvesare also essential because they provide spatial localization forcarbon deposition and nanoribbon formation. Purely thermalprocesses at similar conditions failed, mostly because Ninanobars evaporated faster than graphene nanoribbons wereable to connect the Ni transistor terminals.This example suggests that both nanobar-shaped catalysts

and plasma were essential to enable these unique graphenenanoribbon structures. The graphene could be grown directlyin the device location, without damaging the electrodestructure, which is a common problem in nanoelectronics.Importantly, the nanoribbons showed excellent performance,such as the clear transport gap (present in nanoribbons andabsent in large-area graphene) of 58.5 meV and high on/off

Figure 23. Interaction of the plasma and Ni catalyst during the direct conversion of a Ni nanobar to a graphene nanoribbon. Reprinted withpermission from ref 185. Copyright 2012 Macmillan Publishers Ltd.



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ratio (>104) of transistor devices. This process is scalable andis promising for integration with Si micro/nanofabricationtechnology.Synergistic interactions between the plasma and carbon-

dissolving Ni catalyst also lead to other interesting effects,such as the possibility to grow graphene films on SiO2 surface,which is quite challenging by using thermal CVD or othermethods. Figure 24 shows that it is possible to accomplish

that by depositing a Ni nanofilm (∼55 nm) on SiO2 and thenusing plasma exposure to “push” carbon atoms through the Nifilm to enable nucleation of graphene films at the interfacebetween the Ni film and the SiO2 substrate.

186 Thermal CVDunder similar conditions fails, leading to fast evaporation of Nifilms. The interlayer growth of graphene films only occurswhen the fluxes of delivered hydrocarbon precursors are lowand the energy of the plasma ions is relatively high.Under plasma CVD conditions, using a low flux of carbon

species, and high ion energy, carbon species are able to diffusethrough the Ni layer much faster and reach the Ni−SiO2interface well before the Ni film evaporates, as sketched inFigure 24. Next to the thermodynamics, pertinent to thenucleation in both the thermal-catalytic and the plasma-catalytic growth processes,187 it was suggested that theselectivity of nucleation of graphene at different surfaces/interfaces (or both) also critically depends on the diffusionlength of carbon ions (lc in Figure 24). When the ion energyis higher, lc is larger, and when it becomes larger thanapproximately half of the thickness of the Ni film (TNi inFigure 24), nucleation of carbon atoms at the bottominterface becomes possible. The mechanisms of these plasma−catalyst interactions still need to be understood andquantified. For example, the qualitative criterion thatcompares the diffusion length with the film thickness needs

to be refined to account for the variation of lc with the plasmaprocess (e.g., operating pressure) conditions. Nonetheless, it isclear that both Ni film and plasma exposure are essential toenable nucleation of graphene at the interface between Ni andSiO2, thus evidencing the plasma−catalyst synergistic effect.Graphene growth is possible in another extreme case when

carbon atoms are not or are hardly dissolved in the catalyst.The most typical example is growth of graphene on Cu films.This method has become very popular because of thepossibility to grow high-quality, large-area graphene films,188

and also because of its scalability to roll-to-roll production andtransfer to other substrates for the envisaged applica-tions.189,190 In thermal CVD, such growth typically requiresvery high temperatures of at least 800−900 °C or evenhigher191 and the growth process typically lasts at least a fewtens of minutes. Furthermore, preheating and postprocesscooling are required, costing additional process time andenergy. When the thermal CVD process is carried out incommon thermal furnaces, the combined pressure of thecarbon source and carrier gases is maintained close toatmospheric pressure. Plasma-based methods have emerged asa viable alternative because they allow for the growth atsubstantially lower temperatures, while the working pressure istypically in the order of a few tens to a few hundredmTorr.192−194 Moreover, common CVD processes usuallyrequire postgrowth etching of the Cu catalyst to enablegraphene transfer, which leads not only to the loss of catalystbut also to the need of hazardous chemicals, which raisesfurther environmental issues upon disposal.Lower growth temperatures and faster growth processes can

be regarded as manifestations of synergistic effects of theplasma and the Cu catalyst. When the rates of delivery ofcarbon species are low, the density of the nucleation centerscan be reduced, eventually leading to larger graphene grainsizes. It is noteworthy that this is quite difficult to control inthe plasma because plasmas typically provide high rates ofmaterial delivery to the surface, as is presently wellacknowledged in the literature. To avoid this, it was recentlyproposed to use very low amounts of carbon atoms containedas impurities in Cu foils (and elsewhere in the growthreactor) as carbon source for the graphene growth.195

Another interesting example of synergistic interaction of Cucatalyst and plasmas is in the enabling water-based transfer ofgraphene microwell (GMW) structures.194 The GMWs arehybrid structures made of interconnected layers of horizontaland vertical graphenes, as sketched in Figure 25a. A relativelyshort treatment of polycrystalline Cu foil with H2/Ar plasmasat a very low temperature of 190 °C has led to significantmodifications of the crystal expression on the Cu surfacesuch as the appearance of (101) and (111) domains inaddition to the (100) domain, which was prevailing before theplasma treatment.This plasma-induced modification of the catalyst not only

enables the growth (because higher-index (101) and (111)facets are believed to favor graphene nucleation because ofbetter lattice matching) but also changes the contrast insurface energies and consequently water contact angles of thebottom surface of the GMWs and the top surface of thecatalyst. After the plasma treatment, the Cu foil is hydrophilic,while the graphene surface is hydrophobic (see Figure 25b).This allows intercalating water molecules to adhere to the Cucatalyst and repel the GMW, leading to the observedinteresting effect of water-based GMW film separation.

Figure 24. Mechanisms of graphene growth on top of Ni catalystand at the interface between the Ni catalyst and SiO2 substrate.Reprinted with permission from ref 186. Copyright 2012 AmericanChemical Society.



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The discussed examples of synergistic interactions of theplasma and catalysts are representative and nonexhaustive.The interesting manifestations of their positive practical effectssuggest that this area should definitely be explored further inboth breadth and depth.5.2. Catalytic Growth of Inorganic Nanowires

Metal nanocatalysts are used not only to grow CNTs but alsofor growth of other inorganic one-dimensional materials bothat micrometer and nanoscales. In this case, the growth istypically enabled by the vapor−liquid−solid mechanism.115

The synthesis is typically performed well above the eutectictemperatures to enable rapid mixing and diffusion through themolten catalyst. Various catalytic materials have beensuccessfully used, including Au,196−200 Ni,201 Fe,202 Cu,203

Ag,204 and Co.205 The typical catalytic metals and synthesistemperatures for VLS growth of inorganic nanowires (NWs)are summarized in Table 1. The pressure may range from verylow pressures up to atmospheric pressure.

As shown in Figure 26, Si nanowires are grown by allowingSiCl4 and H2 precursor gases to react on the gold

nanoparticle, leading to selective dissolution of silicon intothe molten gold nanoparticle. The alloying of silicon withgold leads to gold−silicon alloys whose melting point(eutectic temperature) is much less than those of both siliconand gold. The subsequent precipitation from the gold−siliconalloy leads to one-dimensional growth of silicon NWs.Typically, the synthesis temperature for crystalline siliconnanowires on Au tends to be in the range from 550 to 900°C, well above its eutectic temperature.The use of plasma activation of gas phase over traditional

catalytic metal nanoparticles reduces the synthesis temper-atures,206−209 often to near the eutectic temperature. Forexample, this was demonstrated for Au catalyst,210 where thesynthesis temperature has been reduced to 380 °C by usingPECVD. We emphasize that, similar to one-dimensionalcarbon nanotubes described in section 4.1, plasma-catalyticgrowth significantly increases the growth rates of inorganicnanowires.211

An interesting feature of the plasma-assisted processes isthat the temperature of the metallic nanoparticle alloy duringgrowth can be much higher than that of the substrate holderdue to heating by radical recombination and ion impactdissociation reactions.212 Such effects allow for growth at

Figure 25. Growth process of graphene microwells in the plasma.Modification of facet expression on the catalyst surface enables notonly the growth (a) but also water-based film transfer (b).Reproduced with permission from ref 194. Copyright 2014 JohnWiley and Sons.

Table 1. State-of-the-art Catalytic Metals and Synthesis Temperatures for VLS-Grown Inorganic Nanowires; TE is theEutectic Temperature, Tsynth,trad is the Temperature in Traditional Synthesis, and Tsynth,plasma is the Temperature in Plasma-Based Synthesis

material system metallic catalyst reactor class TE (°C) Tsynth,trad (°C) Tsynth,plasma (°C)

Si Au PECVD 363 550−1000196,199 380210

GaN Au DBD,206,208 hot filament PECVD207 461.3,a 55% at. Ga 960200 900,206,208 850207

InxGa1−xN Au PECVD 700−750216

SiOxNy Ni RF-plasma 963,b Si−Ni eutectic 300217

GaN Ni plasma-assisted molecular beam epitaxy 895, Ni−Ga liquid phase 900201 730209

Si Fe arc plasma 1200c 1000202

GaN Co ∼910 800−1050205

Si Ag 836204 490−500204

aElliott, R. P.; Shunk, F. A. The Au−Ga (Gold−Gallium) System. Bull. Alloy Phase Diagrams 1981, 2, 356−358. bMurray, J.; Massalski, T. In Binaryalloy phase diagrams; American Society for Metals: Metals Park, OH, 1986; p 173. cWang, Z. L.; Liu, Y.; Zhang, Z. In Handbook of Nanophase andNanostructured Materials: Synthesis/Characterization/Materials Systems and Applications I/Materials Systems and Applications II; Springer: New York,2003; Vol. 3.

Figure 26. Schematic of silicon nanowire growth using catalyst metalclusters and vapor-phase source for precursors and transmissionelectron microscopy (TEM) image of silicon nanowire grown usinggold clusters through the vapor−liquid−solid (VLS) mechanism.TEM image reprinted with permission from ref 115. Copyright 1964AIP Publishing LLC.



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much lower substrate temperatures. In addition, plasmaactivation enables the growth of thinner nanowires than inthermal CVD (Figure 27).

Importantly, under plasma exposure the supersaturation canbe higher at much lower temperatures owing to the Gibbs−Thomson effect discussed in section 2.2.2. This allows one toproduce nanowires at temperatures below 600 °C and reducetheir lowest possible thickness (size limit), in part due to theenhanced flux of vapor-phase species through the plasmasheath and more effective dissolution.212 Figure 27 shows thatthe critical diameters of Si nanowires for thermal CVD are

larger than those for PECVD. Similar observations have beenmade by other research groups.210,213

The key attributes for plasma activation of nanowiresynthesis include effective dissociation of the precursor gasphase to produce radicals and local heating temperature viaradical recombination, which makes the catalyst temperaturehigher than the average substrate temperature.214 It has beenproposed that electron−ion recombination and other surfacechemical reactions may lead to considerable nanoparticleheating.215 The combined use of gold nanocatalyst andplasmas for the growth of silicon nanowires results in thegrowth of 1D nanostructures at lower temperatures andhigher growth rates than in conventional neutral gas-basedprocesses.210

For the plasma-catalytic growth of GaN nanowires, nitrogencan be used as a precursor instead of ammonia, which istraditionally used in thermal catalysis. This is mainly due tothe fact that the active nitrogen species created by the plasmaallow the growth to happen.208 Indeed, effective dissociationof nitrogen molecules requires energies that are much higherthan typical energies of thermal activation; this becomespossible to achieve in plasmas through electron-impactreactions. The resulting GaN nanowires are single-crystallineand of high quality.Furthermore, the combined use of Au NPs and plasmas has

been shown to result in high-quality single-crystal InGaNnanowires.216 Plasma activation of the gas phase has beenshown to be critical for producing SiOxNy compoundnanowires. In the absence of plasma, one can only synthesizea-SiOxH NWs, regardless of the type of catalyst used.217 The

Figure 27. Critical diameter of catalyst nanoparticle as a function ofthe substrate holder temperature for CVD (solid and dashed curves)and PECVD (dotted and dash−dotted curves) for a pressure p = 50mTorr and a gas composition Ar/SiH4/H2 = 70:20:10. Reproducedwith permission from ref 212. Copyright 2011 IOP Publishing. Allrights reserved.

Table 2. Summary Listing Nontraditional Catalysts and the Synthesis Temperatures Used for Nanowire Growth underPlasma Activation; No Nanowire Growth Is Observed without Using Plasma Activation

materialsystem

metalliccatalyst

metal vapor pressure range (bar),100−800 °C

minimum synthesis temperaturereported (°C)

eutectictemperature (°C)

eutectic solubility(at. %) ref

Si In ∼1 × 10−28−1 × 10−6 240 156 1 × 10−8 aSi In 500 bSi In 400 cGaN In 730 217Si Sn ∼1 × 10−36−1 × 10−9 240 232 1 × 10−7 aSi Sn 380 dSi Ga ∼1 × 10−40− × 10−8 400 219Ge Ga; In 300 30; 156.6 5×10−5; 7×10−3 219,220Ga2O3 Ga 450 3×10−5 @230C 221SiOx Ga 450 eSixNyH Ga 450 eSixGe1−x Ga 500 fSi Ga 220 30 5 × 10−8 219,224Si Ga; Au-Ga 200 gSi Ga 500 h

aYu, L.; O’Donnell, B.; Alet, P.-J.; Conesa-Boj, S.; Peiro, F.; Arbiol, J.; Roca i Cabarrocas, P. Plasma-Enhanced Low Temperature Growth of SiliconNanowires and Hierarchical Structures by Using Tin and Indium Catalysts. Nanotechnology 2009, 20, 225604. bZardo, I.; Conesa-Boj, S.; Estrade, S.;Yu, L.; Peiro, F ; Roca i Cabarrocas, P.; Morante, J.; Arbiol, J.; Fontcuberta i Morral, A. Growth Study of Indium-Catalyzed Silicon Nanowires byPlasma Enhanced Chemical Vapor Deposition. Appl. Phys. A 2010, 100, 287−296. cXie, X.; Zeng, X.; Yang, P.; Wang, C.; Wang, Q. In SituFormation of Indium Catalysts To Synthesize Crystalline Silicon Nanowires on Flexible Stainless Steel Substrates by PECVD. J. Cryst. Growth 2012,347, 7−10. dBall, J.; Bowen, L.; Mendis, B. G.; Reehal, H. Low Pressure Plasma Assisted Silicon Nanowire Growth from Self Organized Tin CatalystParticles. CrystEngComm 2013, 15, 3808−3815. eSunkara, Z. K.; Sharma, S.; Chandrasekaran, H.; Talbott, M.; Krogman, K.; Bhimarasetti, G. BulkSynthesis of a-SixNyH and a-SixOy Straight and Coiled Nanowires. J. Mater. Chem. 2004, 14, 590−594. fMeduri, P.; Sumanasekera, G.U.; Chen, Z.;Sunkara, M. K. Composition controlled synthesis and Raman analysis of Ge-rich SixGe1−x alloy nanowires. J. Nanosci. and Nanotech. 2008, 8, 3153−3157. gCarreon. M. Plasma-Catalysis Using Low-Melting Metals. Ph.D Dissertation, University of Louisville, December 2015. hZardo, I.; Yu, L.;Conesa-Boj, S.; Estrade, S.; Alet, P. J.; Rossler, J.; Frimmer, M.; i Cabarrocas, P. R.; Peiro, F.; Arbiol, J. Gallium Assisted Plasma Enhanced ChemicalVapor Deposition Of Silicon Nanowires. Nanotechnology 2009, 20, 155602.I.



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radical species such as hydrogen and nitrogen are essential forthe synthesis of a-SiOxNy nanowires. In this process, hydrogenradicals keep the catalyst in the reduced form (avoidingexcessive oxidation) while the nitrogen radicals allow fornitrogen incorporation into the synthesized nanowires.In the case of traditional catalyst metals, the use of plasma

activation has been mainly to enhance the growth process interms of growth kinetics, lowering the growth temperature,reducing the size limit, and using different precursors. The useof plasmas has been shown to be critical for growth of one-dimensional materials using nontraditional catalytic metals andsystems. Specifically, metals such as Ga, In, and Sn workeffectively as catalysts for the synthesis of silicon,218,219

germanium,220 and other compound nanowires221 through thevapor−liquid−solid mechanism under the plasma activation.This is particularly important as many of these metals formeutectic alloys with Si and/or Ge at much lower temperaturesthan catalytic metals such as Au and Fe. Specifically, Ga formsthe eutectic alloy with Si at ∼30 °C, and it has a very lowsolubility (∼1 × 10−8 at %) in Si at the eutectic temperature.A peculiarity of these metals is that they are noncatalytic bynature and do not facilitate the growth of 1D materials at anytemperature and pressure. Importantly, several groups haveobserved (without specific mention) the synergistic effectsbetween plasma and normally noncatalytic metals in inorganicnanowire growth. Table 2 summarizes various experimentsinvolving both nontraditional catalysts and plasmas.For the case of GaN nanowire growth with indium catalyst

and plasmas, it has been observed that the much-enhanced Gadiffusion suppresses the formation of the undesired GaN 2Dlayer and dramatically reduces surface contamination.222

Diffusion studies on the growth of GaN nanowires usingAu catalyst and plasmas have shown an interesting interactionbetween Ga and hydrogen plasmas.208 It has been observedthat, when hydrogen is introduced in the chamber, Gadesorbs easily from the surface by reacting with hydrogen.While not acknowledged by the authors,208 this interactionlimits Ga precipitation on the surface (e.g., prevents theformation of the undesired 2D layers mentioned above), in amanner quite similar to hydrogen etching of amorphouscarbon in the CNT/graphene growth examined in sections 4.1and 5.1.Of all low-melting-point metals, Ga has the lowest eutectic

temperature with many materials, including Si, and also hasthe lowest vapor pressure. However, low-melting-point metalssuch as Ga, In, and Sn do not allow molecular adsorption forselective dissolution.219 It has been shown that siliconnanowire growth using gallium can be realized in the presenceof a high density of hydrogen radicals.223 The selectivedissolution of Si derived from the vapor-phase precursorsthrough the plasma activation can be understood byconsidering the various possible surface reactions. Theplasma-generated silyl and hydrogen radicals were found toeffectively interact with molten metal nanoparticles oftraditionally noncatalytic metals such as Ga.Importantly, effective generation of atomic hydrogen in the

plasma enables the synergistic interaction between thenormally noncatalytic Ga nanoparticles and plasmas in theSi nanowire growth. Specifically, in the presence of plasmas,otherwise noncatalytic Ga behaves as a hydrogen sink, leadingto the formation of Ga−H species that mediate thedissolution of the precursors,224 as shown in Figure 28. Itneeds to be articulated that the mild plasma exposure leads to

the activation of the vapor−liquid−solid mechanism ofinorganic nanowire growth and enables the otherwiseineffective catalytic effects in Ga nanoparticles. This is anotherclear example of synergistic effects in plasma nanocatalysis.The synergistic effect of plasma activation can also be

observed in nitrogen dissolution into low-melting metals.Molecular nitrogen is usually a nonreactive gas under mildtemperature and pressure conditions in neutral gas processes.Typically, one requires a pressure of ∼20 atm and atemperature of ∼2000 K for dissolving nitrogen into Gamelts using molecular nitrogen according to reaction 4. Usingplasma activation of nitrogen, the dissolution into Ga melts isfavored at subatmospheric pressures and temperatures as lowas 850 °C according to reaction 5.225

+ ⇌Ga(l) 1/2N (g) GaN(s)2 (4)

+ ⇌Ga(l) N(g) GaN(s) (5)

It was concluded that the recombination of N to form N2 issufficiently slow to allow the formation of GaN.225 Suchinteractions have been exploited to grow GaN nanowiresunder self-catalyzed conditions using plasmas of nitrogen ornitrogen and hydrogen mixtures.As discussed above, the hydrogen radicals resulting from

the plasma can interact with and dissolve into molten metals.Similarly, plasma activation of the gas phase enablesdissolution of both oxygen and nitrogen radicals into moltenmetals. In this case, the direct exposure of molten metals tothe plasma-excited gas can result in oxide, nitride, andoxynitride nanowires. The role of plasma activation here is toenable rapid dissolution of solutes such as oxygen andnitrogen and keeping the surface of molten metals clean. Thisconcept has been successfully exploited for synthesizing oxidenanowires of zinc, gallium, indium, aluminum, and nitrides ofgallium, indium, etc.226

In addition to oxidation or nitridation of low-meltingmetals, it is also possible to use plasmas for producing oxidenanowires from surfaces of high-melting-temperature met-als.227 This method is based on exposing a thin metal foil toreactive oxygen plasmas.228−230 In this case, plasma−surfaceinteraction is the critical step that enables the nanowiregrowth without any external (e.g., through gas feed) supply ofprecursor material. The role of plasmas in this synthesis is togenerate oxygen atoms in the gas phase mostly throughelectron impact, and to provide localized surface heating

Figure 28. Synergistic interactions between plasma and Ga catalystin the growth of silicon nanowires. Reproduced with permissionfrom ref 224. Copyright IOP Publishing. All rights reserved.



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through recombination of the dissociated oxygen atoms, aswell as ion bombardment and neutralization.Another method involving plasma oxidation of metals or

metal oxides in the presence of alkali salts results in nanowireseven after a very short exposure of <1 min.231 A schematic ofplasma oxidation of metals in the presence of alkali salts fornanowire growth is shown in Figure 29. The resulting alloys

melt at lower temperatures and enable nucleation and growthof high-density nanowires. This technique is similar tohydrothermal growth except that the use of plasmas reducesthe time scales from several days to several minutes or evenshorter. An overview of the time scales involved is shown inFigure 30, indicating that plasma-based techniques featureshort reaction times, making them potentially amenable forhigh-yield, industrial-scale production.232

5.3. Abatement of Toxic Waste and Air Pollution Control

The synergy of plasma and catalyst in toxic waste abatementis clearly demonstrated in Figure 31. This figure shows thedestruction efficiency of 500 ppm toluene in air, by means ofplasma, thermal catalysis (using a Ag/Al2O3 catalyst), and thesum of both individual processes, as well as plasmacatalysis.233,234 The sum of the two separate processes isonly 20%, whereas the plasma-catalytic destruction efficiencyis 65%, hence indicating a synergistic gain of more than afactor of 3.Many other papers have shown such a synergy for plasma

catalysis in air pollution control, and several review papersoverview the type of catalysts used, the pollutant treated, thedischarge conditions used, etc.80,83,85,235,236 Given the natureof the problem, the pressure in these experiments is invariablyatmospheric pressure. The most widely used catalyst supportsare TiO2 and γ-Al2O3. Moreover, a variety of other metaloxides (e.g., MnOx, V2O5, Fe2O3,CuO, WO3, ...) as well as

metals (e.g., Ag, Ni, Pt, Cu, ...) have been loaded as activeelements on these supports.83,236 It is difficult or impossible todraw a definite conclusion on the most suitable catalyst, basedon all the studies available in literature. Likewise, theunderlying reasons for the synergistic effects are not yetcompletely understood. Several review papers try to explainthe performance enhancement of plasma catalysis, based onthe effects of the plasma on the catalyst, and vice versa,summarizing the many studies presented in litera-ture.80,83,85,236 Similar explanations, but more from afundamental perspective, were also given in sections 3 and 4.For photocatalysts, like TiO2, some papers state that UV

light from the plasma can create electron−hole pairs.81,237

Sano et al. demonstrated that, while UV light was produced inthe absence of TiO2, no emission was detected when TiO2was present, suggesting that the UV light is effectivelyabsorbed by the catalyst.81 On the other hand, several otherpapers claim that the intensity of UV light from the plasma isnot high enough.80,238,239 Indeed, the UV dose in typicalphotocatalytic processes should be in the order of severalmW/cm2, whereas in typical (air) plasmas it is only in theorder of μW/cm2.80 However, it is very possible thatphotocatalysts are activated by other (energetic) plasmaspecies, like ions, metastables, or electrons with suitableenergy. Indeed, TiO2 photocatalysts have a typical bandgap of3.2 eV; hence, electrons in the plasma with energies of ∼3−4eV should be able to excite electrons to the conduction bandand to create electron−hole pairs in a similar way as producedby UV light.239 The same mechanism was also recentlysuggested for BaTiO3 photocatalysts.

168 Similarly, Van Durmeet al. explained the activation of a TiO2 catalyst surface by theadsorption of metastable N2, having an energy of 6.17 eV.83

Hence, we may conclude that the plasma might directly affectcarrier generation in (photo)catalysts, and this might explainthe synergy in plasma catalysis when using photocatalysts.Finally, as mentioned in section 4.2.3, it is important to

make a distinction between the physical and chemical effectsin plasma catalysis. The physical effects even can be due tononreactive materials packed in the plasma reactor to supportthe catalyst. For example, it has been demonstrated that apacked-bed DBD, even with noncatalytic dielectric beads, canincrease the energy efficiency up to a factor of 12, depending

Figure 29. Rapid plasma oxidation of metals in the presence of alkalisalts for producing nanowires. Reprinted with permission from ref231. Copyright 2011 American Chemical Society.

Figure 30. Time scales for the plasma-based techniques for scalableproduction of inorganic nanowires. Reproduced with permissionfrom ref 232. Copyright IOP Publishing. All rights reserved.

Figure 31. Destruction efficiency of 500 ppm toluene in air at 25 °C,by means of DBD plasma (P), thermal catalysis with Ag/Al2O3catalyst (C), and the sum of both individual processes (P+C), as wellas plasma catalysis (PC), illustrating the synergistic effect of theplasma catalysis. Data adopted from ref 233.



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on the type of pollutant being treated, the reactor geometry,and the packing pellets, compared to a nonpacked DBDreactor.235 Moreover, the shape of nanosized particles canenhance the plasma catalysis (see sections 2.2.2 and 3.3). Kimand co-workers have demonstrated the physical role of nm-sized metal particles (Ag, Zr, Cu) for discharge generationover the surface of zeolites.240 The plasma expansion occurredover a wide surface area, whereas with bare zeolites theplasma generation was limited to the edges of the zeolites.This enhanced plasma expansion was correlated to a betterVOC decomposition.240

5.4. Hydrocarbon Reforming

Hydrocarbon reforming has been receiving immense attentionglobally because it is poised to become a major source ofhydrogen, a product that has the potential of emission-freealternative fuel. Currently, almost 90% of the H2 is producedvia high-temperature steam reforming of natural gas or thelight oil fraction.241 Although not widely used today as atransportation fuel, researchers are working toward the goal ofclean, economical, and safe hydrogen production and fuel-cellelectric vehicles.Even though catalytic hydrocarbon reforming has about

seven decades of history, there are still some inherentchallenges such as moderate thermodynamic efficiency;fouling/coking on catalyst surface and eventual deactivation;poisoning of a certain catalyst as a result of sulfur impuritiesin hydrocarbon feedstock; and high temperature requirementsof the reforming reaction (a condition that reduces the energyefficiency of the process). The reaction pathways in thereforming process generally depend on the oxidant used. Forthe wet reforming process, steam oxidizes CH4 to CO (eq 6).Equation 7 is the water gas shift (WGS) reaction, oxidizingCO further to CO2. The overall reaction is represented by eq8. Clearly, the energy requirement (endothermic, 165 kJ/mol)of the overall process is high, which makes the wet reforminga less favorable route.

+ ⇌ + Δ = · −HCH H O CO 3H , 206 kJ mol4 2 2 2981

(6)

+ ⇌ + Δ = − · −HCO H O CO H , 41 kJ mol2 2 2 2981

(7)

+ ⇌ + Δ = · −HCH 2H O CO 4H , 165 kJ mol4 2 2 2 2981

(8)

Plasma reforming has a number of advantages overtraditional reforming,242 viz. (1) compactness, (2) highconversion efficiency, (3) reasonable cost, (4) short residencetime, and (5) suitability for a broad range of hydrocarbons,including heavy oil, raw biofuel, and other difficult-to-usefuels. Most often, a DBD reactor at atmospheric pressure isused, as was the case in plasma-catalytic DRM (see section4.2.3). Plasma reforming of methane is an energy-efficientprocess with significant potential for industrial applications.Bromberg et al. reported a hydrogen yield of 100%corresponding to a methane conversion of 80%.242 Thisresult was achieved through heat regeneration and efficientuse of heat exchangers and further optimization.The effect of initial water content on steam reforming of

aliphatic hydrocarbons with nonthermal plasma was analyzedin terms of hydrocarbon conversion, carbon recovery, andproduct selectivity.243 It was found that water additionincreased the CO2 yield, despite a decrease in hydrocarbonconversion. The number of carbon atoms in the hydrocarbon

affected the effect of water addition due to the insufficientsupply of oxygen atoms from water.Futamura et al. used a ferroelectric packed-bed reactor as a

standard reactor in the plasma reforming of methane,propane, and neopentane with both H2O and CO2.

244

Methane and propane were found to be more suitable forreforming. Achieving a constant composition of syngas bycontrolling the ratio of initial concentrations of hydrocarbonand oxidant is possible. We stress that in this case the plasma-catalytic effect is predominantly physical because no clearchemical pathways have been activated through the plasmaexposure.Nozaki et al. clearly demonstrated a synergistic effect for

plasma-catalytic CH4 steam reforming with a Ni/SiO2 catalyst,as shown in Figure 32.245 The conversion in the DBD plasma

without catalyst (blue solid line) was ∼22% independent oftemperature, but above 400 °C, solid carbon (or soot) wasgradually accumulated and a stable plasma could not bemaintained. The catalytic conversion without plasma (greenline) was negligible at temperatures below 400 °C butincreased at higher temperatures, especially above 600 °C.Finally, the conversion in the plasma-catalysis setup (red line)was already near 25% at 200 °C and increased to almost 90%at 700 °C.Note that the plasma-catalytic CH4 conversion largely

exceeds the equilibrium conversion (indicated by the bluedashed line). The most pronounced synergistic effectsbetween plasma and Ni/SiO2 catalyst take place between400 and 600 °C. Furthermore, the CH4 conversion curveshifts to a lower temperature by ∼200 °C in the plasma-catalytic setup compared to the thermal-catalysis case (cf. redand green curves). Hence, the plasma significantly reduces theactivation temperature of the catalyst, which is beneficial interms of energy efficiency. Indeed, the energy efficiencyclaimed in this process was 69%.245

This synergy may be attributed to the presence ofvibrationally excited CH4 molecules (see section 3.4),produced in the plasma.73,95 In plasmas without catalyst,these vibrationally excited CH4 molecules are ineffective inhydrocarbon reforming, because of their short lifetimes (orderof ns) and low threshold energy (below 2 eV, whereas thebonding energy for most hydrocarbons is between 3 and 6eV). Hence, the energy residing in the vibrationally excited

Figure 32. CH4 conversion in a DBD (blue solid line), Ni/SiO2catalyst (green line), and combined DBD with Ni/SiO2 catalyst (redline); the blue dashed line indicates the equilibrium conversion at thegiven temperature. Reprinted with permission from ref 245.Copyright 2004 Elsevier.



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states is not efficiently used in this case; it was claimed that40% of the plasma power is wasted in this way.73,95

Although the vibrationally excited CH4 molecules are notreactive in the plasma, they appear to be much more reactiveat a Ni surface than ground-state CH4 molecules.246 Forinstance, eigenstate-resolved measurements demonstrated thatthe antisymmetric ν3 C−H stretch vibration yields a CH4reactivity on a clean Ni(100) surface 1600 times highercompared to the molecule in the ground vibrational state.36,37

Indeed, in the case of plasma catalysis, vibrational excitationimproves dissociative chemisorption of CH4 molecules on acatalyst surface at low temperatures (see Figure 8),82 leadingto a significant process enhancement. In other words, the CH4conversion largely exceeds the equilibrium conversion at thegiven temperature. Note, however, that the subsequentchemical reactions occur at the catalyst surface, independentof the plasma; hence, the product selectivity does followchemical equilibrium.73 Finally, physical effects of the plasma,i.e., streamer impact, are claimed to be beneficial, as they mayheat the catalyst bed.73

A similar synergy also holds for CH4 steam reforming withcommercial 12 wt % Ni/γ-Al2O3 catalyst in a packed-bedDBD.95,247 Besides the vibrationally excited CH4 molecules(see above), the heat produced by the DBD plasma enhancesthe catalytic conversion of CH4. Furthermore, kinetic analysisrevealed that the rate-determining step of the process is thedissociative adsorption of methane (see section 2.1.1).Arrhenius plots for the catalytic reaction with and withoutDBD suggest that the activation energy is very similar in bothcases. The pre-exponential factor, however, indicative for thecollision frequency (or number of active sites available), was50 times higher (in the reaction-limited regime) in the case ofthe plasma-catalytic conversion. Indeed, to maximize the CH4conversion in steam reforming, dehydrogenation of CH4 andchemisorption of H2O molecules should be promotedsimultaneously. In this way, one can oxidize C intermediatesand thus prevent blocking of the catalytically active sites.Hence, the excited H2O molecules play a vital role, as theyincrease the concentration of chemisorbed O and OHradicals. These radicals are needed for oxidation (anddesorption) of the chemisorbed C intermediates, therebyleaving sufficient active sites available for CH4 dehydrogen-ation.95,247

Some other examples suggest synergistic enhancement ofhydrocarbon reforming by a factor 1.1−2.4 through plasmacatalysis.82 This enhancement is likely to be related tovibrational excitation of CH4 molecules in the case of plasma-catalytic hydrocarbon reforming. Indeed, pure plasma-basedhydrocarbon reforming, without catalysis, is energy-intensive,as the conversion needs to proceed through dissociation andionization of the molecules. As discussed earlier, vibrationallyexcited molecules have a low energy and short lifetime to playany significant role in the gas-phase conversion. Theminimum threshold for dissociation and ionization of CH4is ∼9 eV. Therefore, the energy cost for plasma-based (steam)CH4 reforming is ∼10−100 eV.82 This is much higher thanwhat is needed for thermal catalysis, i.e., the energy needed toincrease the CH4 gas temperature from 25 °C to the typicalworking temperature of 700−900 °C is ∼0.38−0.53 eV.However, in plasma catalysis the energy can be channeled

into vibrationally excited molecules, thereby enhancing theirreactivity and the overall catalytic effects. While it wasconcluded that hydrocarbon reforming in plasma is predom-inantly thermal catalysis, the plasma assists in providing theinternal energy of the reactants (i.e., so-called “plasma-assistedcatalysis” instead of “catalysis-assisted plasma conversion”).82

It should be realized, however, that the efficiency ofvibrational excitation in enhancing the dissociation is notindependent of the catalyst, and this division is probably not acorrect assumption.The extent of vibrational excitation also varies among the

gas discharges. As was mentioned in section 4.2.2, DBDplasmas produce only limited vibrational excitation. Note thatin warm (i.e., microwave or gliding arc) plasmas vibrationalexcitation is more prominent, but the combination withcatalysis is still rather unexplored. However, vibrationalexcitation in DBDs can be promoted by using alternatinghigh-voltage and dc pulses.82 Furthermore, as the lifetime ofthe vibrationally excited molecules is only in the order of ns,these excited molecules must be delivered to the surface asquickly as possible. An improved design of the plasma-catalytic DBD reactor based on perforated electrodes coatedwith catalysts improves the interaction of the gas flow withthe catalyst by both increasing the surface contact areas andaligning the catalyst surface perpendicular to the direction ofthe gas flow.82

Table 3. Performance of Selected Plasma and Plasma + Catalyst Reforming Processes

hydrocarbon reforming type reactor H2 yield (%) T (°C) plasma power (kW) ref

CH4 steam plasma only 44 20 0.025 244CH3C(CH3)2CH3 CO2 plasma only 17 20 0.025 244CH4 O2 plasma only 32 400 0.033 248iso-octane O2 plasma only 0 400 0.008 aC2H5OH steam plasma only 63.9 350 0.009 bCH4 (biogas) CO2 plasma/catalyst (NiO/Al2O3) 59 700 2.4 cC2H5OH steam plasma/catalyst (Pt/TiO2) 73.5 300 0.009 bCH4 O2 plasma/catalyst 89.9 750 0.0324 250CH4 O2 plasma/catalyst (NiO/α-Al2O3) 72 400 0.033 248CH4 CO2 plasma/catalyst (Ni/γ-Al2O3) 9.6 230 0.06 100iso-octane O2 plasma/catalyst 9 800 0.0038 a

aSobacchi, M. G.; Saveliev, A. V.; Fridman, A. A.; Kennedy, L. A.; Ahmed, S.; Krause, T. Experimental Assessment of a Combined Plasma/CatalyticSystem for Hydrogen Production via Partial Oxidation of Hydrocarbon Fuels. Int. J. Hydrogen Energy 2002, 27, 635−642. bZhu, X.; Hoang, T.;Lobban, L. L.; Mallinson, R. G. Low CO Content Hydrogen Production from Bio-Ethanol Using a Combined Plasma Reforming−Catalytic WaterGas Shift Reactor. Appl. Catal., B 2010, 94, 311−317. cChun, Y.N.; Yang, Y. C.; Yoshikawa, K. Hydrogen Generation From Biogas Reforming UsingA Gliding Arc Plasma-Catalyst Reformer. Catal. Today 2009, 148, 283−289.



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Another example was presented by Pietruszka andHeintze,248 where the DBD reactor was employed for thesteam reforming of methane at temperatures below 400 °C.When the discharge was used solely, only methane andoxygen were converted. However, by combining the DBD anda Ni catalyst, the conversion of methane was not improvedbut complete oxygen conversion was achieved. At a sufficienttemperature to maintain the Ni catalytically active, theproduct selectivity changed significantly. The effect of thesteam led to enhanced hydrogen yield, provided that oxygenwas fully converted. A selectivity for H2 production of ∼70%over NiO/Al2O3 was achieved. It was found that plasma onlyactivates the reagents and thus accelerates the adsorption−desorption processes, whereas the oxidation state of thecatalyst is responsible for the surface reactions. Theperformance of more combined plasma/catalyst reformingsystems is summarized in Table 3.Partial oxidation of methane

+ ⇌ + Δ = − · −HCH 0.5O CO 2H , 35.7 kJ mol4 2 2 2 2981

(9)

is suggested as a suitable alternative to alleviate the energycost.249 Chao et al. reported hydrogen production throughpartial oxidation of methane by combining the arc plasmawith catalysts.250 Because the partial oxidation reaction ishighly exothermic, no additional energy was needed tomaintain the temperature of the catalyst bed. Therefore, theenergy of the exothermic reforming combined with the energyfrom the hot thermal plasma is sufficient to sustain theprocess, leading to a remarkable energy efficiency of 1.21 MJ/kg of hydrogen, a hydrogen yield of 89.9%, and a methaneconversion in excess of 90%.Rico et al. presented convincing results on the performance

enhancement due to the plasma, in plasma-catalytic methanolreforming.251 The authors used a BaTiO3 ferroelectric packingand a Cu/Mn oxide catalyst (mainly consisting ofCu1.5Mn1.5O4 spinel structure with cubic symmetry and aminority phase of Mn2O3), which was mixed with the BaTiO3packing for a uniform distribution. The temperature was keptbetween 115 and 180 °C to avoid condensation of methanol(or H2O, in the case of steam reforming). An ac voltage of800 V was applied, yielding a power of 4−12 W in thepacked-bed DBD. Note that, without packing, an appliedvoltage of 3 kV was needed, yielding 60 W power,demonstrating again that a packing can reduce the voltageto sustain the plasma and, hence, yield better energyefficiency. As illustrated in Figure 33, no methanol conversionwas detected in the case of the BaTiO3 packing withoutplasma. Likewise, the BaTiO3 packing with Cu/Mn oxidecatalyst, but without plasma, yielded a conversion of only 2%.The formed products were formic acid, formaldehyde, andwater.251

However, after plasma addition, the methanol conversionincreased drastically. A maximum conversion of 92% wasobtained in the case of the BaTiO3 packing with plasma,producing only syngas (CO + H2). This clearly indicates thatthe methanol conversion at a temperature of 115 °C is purelyplasma-induced. The intended application of this process isH2 production for fuel cell applications.As the CO can poison the Pt electrode, it should be

avoided, and this could be realized by adding the Cu/Mnoxide catalyst for the selective oxidation of CO into CO2. Inthe combined plasma-catalysis system with BaTiO3 packing,

Cu/Mn oxide catalyst, and DBD plasma, a maximumconversion of 97% could be reached, producing CO, H2,CO2, and H2O (see Figure 33). The CO selectivity wasreduced to 36%, while a H2 selectivity as high as 80% wasachieved. At higher flow rates, the conversion decreases whilethe selectivity remains unchanged.251

5.5. Ammonia Production

The current industrial ammonia synthesis uses the thermalHaber-Bosh technology developed over 100 years ago.252 Thisprocess (eq 10) takes place at temperatures around 500 °Cand pressures of 300 bar. Nitrogen is drawn from theatmosphere by using cryogenic air separation while hydrogenis mostly sourced from natural gas.

+ ⇌ Δ = − ·

Δ = − · ·

−

− −

H

S

3H N 2NH , 92.22 kJ mol ,

198.76 J K mol2 2 3 298

1

2981 1

(10)

Since the reaction is exothermic, eq 10 shows that, due tothe entropy change, the reaction will only proceedspontaneously at sufficiently low temperatures. However, thenitrogen dissociative adsorption presents a high activationbarrier (dissociation energy of 911 kJ/mol of the triplenitrogen bond). On the other hand, the rates of ammoniasynthesis increase with increasing temperature until amaximum is reached. A further temperature increase resultsin the ammonia decomposition, lowering the equilibriumconcentration of ammonia.253 Hence, the process temperatureand pressure can be balanced to maximize the ammonia yield.In a practical process, multiple reduced Fe3O4 catalyst bedsare placed in series, leading to an energy consumption of9500 kWh per ton of NH3 if H2 is generated from steamreforming of methane.At the molecular level, the ammonia synthesis follows a

Langmuir−Hinshelwood mechanism (briefly discussed insection 3.2.3), where the reactants undergo a dissociative

Figure 33. Schematic of the methanol conversion by BaTiO3 packingwithout plasma (A), BaTiO3 + Cu/Mn oxide without plasma (B),BaTiO3 packing with plasma (C), and BaTiO3 + Cu/Mn oxide withplasma (D). The obtained conversions and products formed are alsoillustrated. Adopted with permission from ref 251. Copyright 2009The Royal Society of Chemistry.



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adsorption on the catalyst surface before any reaction betweenthem. The various elemental reactions and the correspondingenergy diagram are presented in Figure 34.254

Plasma-catalytic methods are pursued as promisingtechniques for ammonia synthesis because they provide activenitrogen species at relatively low temperatures. A summary ofrecent results is given in Table 4. Note that, in contrast to thehigh pressures used in traditional thermal ammoniaproduction, plasma-catalytic studies have employed pressuresin the range from 20 Pa to 1 atm.The active nitrogen species may interact on the catalyst

surface, overcoming the energy barrier associated withnitrogen dissociative adsorption.255,256 Nonequilibrium plasmais capable of generating NH radicals from a N2−H2 mixture inorder to obtain ammonia. Besides, the metals placed in theplasma region enhance the ammonia yield, clearly indicating asynergistic effect of the plasma and the catalyst.256

Mindong et al. reported the use of a microgap dielectricdischarge operating at ambient pressure for ammoniasynthesis.257 The maximum yield of ammonia reportedreached 12 500 ppm (1.25%). A later study showed thatthe ammonia production can be maximized by optimizing thecomposition of gas mixtures and sequential inlet of specific

gases into the reactor chamber directly into the reaction zoneof discharge afterglow.258

Species formation in an electron cyclotron resonance(ECR) plasma and at a catalyst surface was alsoinvestigated.259 The nature of the catalytic surface affectedthe production and surface adsorption of the plasma-activatedspecies of diatomic nitrogen, ionized diatomic nitrogen, andNHx. Once the species are adsorbed on the catalyst, hydrogenreacts with them to form NHx, which leads to ammoniadesorption.259 Uyama et al. have demonstrated a synergisticenhancement in ammonia synthesis when Fe wires are used ascatalyst in the presence of plasma. Ammonia production withplasma catalysis is doubled over a period of 3 h under 5 Torrof pressure and 620 ± 50 K. The plasma catalysis results in 2orders of magnitude higher yields than the process withoutthe discharge.260

When the plasma is rich in radicals, especially NH (e.g.,under microwave excitation), NH3 is predominantly formed.This can be explained by noting that mostly nitrogen atomand/or metal−nitrogen bonds are formed on the catalystsurface. When ionic species are abundant (e.g., by RFexcitation), both ammonia and hydrazine are formed. Thiscan be explained by the fact that in this case hydrogenatednitrogen species are detected more abundantly on the catalystsurface than nitrogen atoms and/or metal−nitrogen bonds.261

Van Helden et al. demonstrated a strong correlationbetween ammonia formation and the fluxes of atomic Nand H radicals to the surface.262 Moreover, catalyst playedlittle effect under strong radical flux conditions, which furthersupports the importance of radical generation in the gas phaseof the plasma.Using a membrane-like catalyst reactor, Mizushima et al.

studied the effects of input voltage, hydrogen-to-nitrogenmolar ratio, plasma composition, and metal loading on analumina support.263 The latter consisted of Ru, Pt, Ni, and Fe,and among these tested metals, Ru showed the highestactivity in the plasma-catalytic synthesis of ammonia. Figure35 illustrates the synergistic effects of plasma catalysisreported by Mizushima et al.263

From their findings, they suggest that the nitrogen isadsorbed onto the alumina surface by dissociative adsorptionor diatomic plasma excited diatomic molecules attaching tothe alumina surface. Meanwhile, plasma-generated H atomsand diatomic excited hydrogen can react directly withnitrogen atoms that are adsorbed on an alumina surface or

Figure 34. Reaction steps of the catalytic ammonia synthesis andcalculated reaction pathways for ammonia synthesis over flat (dashedline) and stepped (solid line) Ru surfaces. Empty catalyst surfacesites are represented as *, and X* is the X species adsorbed.Reprinted with permission from ref 254. Copyright 2006 AmericanChemical Society.

Table 4. Reports on Plasma-Activated Catalysis for Ammonia Synthesis; RT Stands for Room Temperature

reactor T (K) P N2/H2 production ref

microwave discharge/Fe wires − Mo wires 620 K 650 Pa aECR plasma/steel RT 600 Pa 1:3 259RF discharge/Fe wires 620 K 5 Torr 1:4 260RF discharge − microwave discharge/Fe wires − Mo wires 620 K 5 Torr 1:4 261expanding thermal plasma/stainless steel−SiNx 20 Pa 11% of outlet 262thermal plasma/stainless steel 600 K 20 Pa 2% of outlet belectric field discharge/MgO ambient 105 Pa 1:4 5000 ppm cDBD/alumina (Ru) RT 1 atm 1:3 1.6 × 10−5 mol·min−1 (2.4% conversion) 256DBD/alumina (Ru, Pt, Ni, Fe) ambient ambient 1:3 3 × 10−5 mol·min−1 263

aMatsumoto, O. Plasma Catalytic Reaction in Ammonia Synthesis in the Microwave Discharge. J. Phys. IV France 1998, 08, Pr7−411 and Pr417−420. bVankan, P.; Rutten, T.; Mazouffre, S.; Schram, D. C.; Engeln, R. Absolute Density Measurements of Ammonia Produced via Plasma-ActivatedCatalysis. Appl. Phys. Lett. 2002, 81, 418. cMingdong, B.; Xiyao, B.; Zhitao, Z.; Mindi, B. Synthesis of Ammonia in a Strong Electric Field Dischargeat Ambient Pressure. Plasma Chem. Plasma Process. 2000, 20, 511−520.



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go through an absorption step first and then react. As for therole of Ru atoms, Mizushima et al. propose that hydrogen isabsorbed on these active sites and the Ru−H species reactrelatively faster with adsorbed nitrogen on alumina, andtherefore, a higher ammonia yield is obtained, againdemonstrating the plasma-catalytic effect.263

6. CONCLUSIONS AND OUTLOOKThe recent advances in the areas discussed in this reviewsuggest that plasma catalysis is a rapidly developing researchfield, which promises not only many exciting scientificdiscoveries but also the development of new technologiesand industrial applications. Chemical catalysis that relies oncatalytic effects of nanoparticles and other nanomaterials hasmany established applications, ranging from natural gasreforming into syngas, the growth of inorganic nanowiresand nanotubes, and ammonia production, to name but a few.As many examples in this review suggest, the interaction

between catalysts and low-temperature nonequilibrium plasmamay lead to synergistic effects. These effects may improve theprocess outcomes, for example, in the conversion and energyefficiency and selectivity of the process, and may potentiallylead to outcomes that are normally not achievable usingtraditional catalytic or plasma-only approaches separately.The outcomes may improve existing catalytic processes of

technological importance and are subject to further explora-tion, and may potentially lead to novel catalytic processes thatcapitalize on the unique outcomes of the synergistic plasma-catalyst effects. In spite of the demonstrated advantages,however, plasma catalysis also faces several challenges, bothscientific and technological, on the way to widespreadpractical adaptation of these unconventional catalyticapproaches. On the other hand, these challenges createexciting opportunities to further improve our scientificunderstanding of the fundamental phenomena involved andeventually translate the new knowledge into viable processesand products.Below we briefly discuss the arising challenges and

opportunities for the plasma-catalysis field in the comingyears. Without trying to be exhaustive, we will follow theselected areas of this review where the synergistic effects ofthe plasma and catalytic materials are particularly clear. Theseareas include (i) plasma-assisted catalytic reforming of naturalgas into syngas; (ii) plasma-based large-area graphenesynthesis; and (iii) large-scale production of multipurposeinorganic nanowires.In any area of applications, including the ones mentioned

above, using plasmas necessarily implies additional costs dueto specialized equipment and energy needed to generate andmaintain the plasma. This creates a perception that plasma-

based applications are energy-intense and costly, especiallyupon industry-scale operation.However, the overwhelming success of the multibillion-

dollar semiconductor industry, where capital-intense, low-pressure plasma nanofabrication facilities are used, suggeststhat the benefits of the creation of value-added products thatotherwise are difficult or even impossible to achieve mayoutweigh the need for the major capital investments inequipment and infrastructure and the associated energy costs.Let us discuss this issue for the selected focused examples

of plasma-assisted catalysis. Generally speaking, theseexamples show value-added outcomes by using similar orsimpler equipment and processes than those in the semi-conductor industry. Moreover, additional energy costsassociated with the plasma generation may be offset by thebenefits offered by the synergistic plasma-catalytic processes.These benefits are expected to be retained at large-scaleoperation, which, however, introduces specific challenges.Specifically, in the first example related to plasma-assisted

catalytic reforming of natural gas, possible important benefitsinclude operation at reduced temperature, selectivity towardthe formation of value-added products, and an extremely fastswitch-on time. Operation at reduced temperature leads toreduced coke formation and catalyst poisoning and, therefore,to increased catalyst lifetime. This is a critical parameter inany industrial setting, in part offsetting the capital costs.Although much research is still needed, plasma-catalytic

reforming of natural gas may allow the selective production ofsyngas, but possibly also other value-added products such asmethanol or formaldehyde. This is due to the very nature ofthe plasma, which in part decomposes and activates theprecursor gases, while the catalyst allows for the selectiveformation of desired products.The fast plasma switch-on time finally may allow to

(partially) cover the imbalance between supply and demandof energy and, hence, the use of intermittent excess energy,e.g., from renewable energy sources, in order to store thisexcess electrical energy in the form of liquid fuels. There arealso many challenges associated with this application. Clearly,large-scale gas conversion requires significant energy input,and hence, the energy efficiency of the process is critical. Themajor challenge, therefore, is to improve this energy efficiencyand reduce the energy cost as much as possible.A possible way to overcome this challenge is to deposit the

appropriate amount of plasma energy in the required energymodes. Specifically, plasma-induced selective pumping of themolecular vibrational levels may provide a means to allow forincreased decomposition at the catalyst surface, withoutwasting energy on heating the process gas.Achieving this, however, requires a thorough understanding

of how the plasma power is coupled into these modes andhow these modes in turn give rise to the productintermediates, which are favorable for generating the desiredend-product molecules. The advent of combined high-levelsimulations and dedicated experiments will be of paramountimportance in this field.In the second example related to plasma-based, large-area

graphene synthesis, the growth of graphene films is presentlypossible on much larger substrates than those discussed insection 5.1. For example, few- and single-layer graphene filmshave recently been synthesized on large-area Cu foil substratesusing microwave plasmas.195,264 The obvious benefits of theseprocesses include amenability for scaled roll-to-roll processing,

Figure 35. NH3 formation pathway on Ru-membrane alumina inN2/H2 plasma. Reproduced with permission from ref 263. Copyright2007 Springer Science and Business Media.



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substantially reduced (compared to conventional thermalCVD) substrate and gas temperatures, and the possibility toproduce good-quality graphene films using low amounts ofcarbon precursor or even without using carbon-containinggases.The remaining challenges include the need to further

improve graphene film quality and to increase the size ofdomains with single-layer structure from typically presentlyachievable ∼100 nm in plasma-based processes to micro-meters and beyond, reported in conventional thermal CVDyet at much higher temperatures. Important issues includingcatalyst reuse and reducing the energy consumption needed tomaintain plasmas and vacuum operation should also beaddressed.These issues can be overcome by more precise control of

the growth and separation of graphene from the metalsubstrates, which should ideally be reused without losing theircatalytic and growth-support functions. The energy efficiencyof the plasma-based processes could be improved by moreefficient precursor gas handling (e.g., dosing and recycling),using cheap, minimally processed precursors, minimizing theamounts of carbon-bearing and other process gases, andgenerating plasma discharges featuring time-programmed (e.g.,pulsed) energy-delivery functions.A better understanding of the fundamental mechanisms of

the interaction of plasmas with graphene-supporting catalystsis therefore needed, for example, to increase the graphenedomain sizes well beyond what is presently possible at lowtemperatures typical for plasma-based processes. Separation ofgraphene films from the catalyst in water without etching ordamaging the catalyst still remains poorly understood.265

A clear explanation of the microscopic effects of the plasmatreatment on the catalyst that enables this separation willmake it possible to develop reliable strategies for the scaledproduction of graphene films on reusable metal catalysts. Aswas explained in section 5.1, synergistic plasma and catalysteffects enable microscopic surface modifications that not onlyare beneficial for graphene growth but also enable the filmseparation in water. The obvious challenge is to demonstratethe feasibility of this process at large (e.g., roll-to-roll)processing scale. Combined experimental and numericalmodeling efforts are expected to shed more light on theseinteresting synergistic plasma-catalytic effects.In the third and final example of large-scale production of

multipurpose inorganic nanowires, the benefits of usingplasma catalysis include a short reaction time scale, whichallows for the large-scale production of nanowires as powders,and similarly short reaction time scales and lower temper-atures for growing inorganic nanowire arrays on large areasand substrates such as thin metal foils, flexible plastic, andpaper substrates, which moreover are also amenable for large-scale processing.While the scaling-up is already in progress, various

challenges remain. On a practical level, there are still manyunknown aspects about equipment design for scale-up. Therequired design studies, however, are both costly and time-consuming. Similar to plasma-catalytic gas reforming, thecapital costs are likely to be significant. On a morefundamental level, the structure, properties, and functionalityof catalysts used under plasma activation need to beunderstood for specific materials of interest. This, in turn,requires better understanding of the elementary processesinvolved in plasma−catalyst interactions.

In terms of nanowire powder production, the requiredcapital cost may be offset by the possible lowering of theactual production cost, which may be achieved throughincreasing throughput, potentially by using fluidized-bedreactors and atmospheric plasmas. In the case of nanowirearray or film production, the methods need to be optimizedfor large-scale (e.g., roll-to-roll) processing.As we have seen from these examples, a substantial research

effort is needed to improve both the fundamental under-standing of the numerous physicochemical effects involvedand the means of translating these effects into practicaloutcomes. This makes the plasma-catalysis field both excitingand challenging, especially for multidisciplinary collaborations.Finally, we expect more enthusiastic researchers to engage inthe coming years and welcome any critical discussions andcollaborations.

AUTHOR INFORMATIONCorresponding Author

*E-mail: [email protected]

The authors declare no competing financial interest.

Biographies

Erik C. Neyts is professor at the University of Antwerp, Belgium. Heis an expert in the field of atomic-scale modeling and simulation ofnanostructures and plasma−surface interactions. He has authoredover 90 peer-reviewed papers, including 6 invited papers, and a bookchapter on invitation. He has given 25 invited presentations atinternational conferences and over 20 invited seminars. He served asguest editor for a special issue in J. Phys. D: Appl. Phys. (2014) on“Fundamentals of Plasma−Surface Interactions” and for a specialissue in Catal. Today (2015) on “Plasmas for Catalysis”.



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mailto:[email protected]


Kostya (Ken) Ostrikov is a Science Leader of the Office of ChiefExecutive with CSIRO, and a Professor with Queensland Universityof Technology, Australia. His achievements include Pawsey (2008)medal of Australian Academy of Sciences, Walter Boas (2010) medalof Australian Institute of Physics, Building Future Award (2012),NSW Science and Engineering Award (2014), election to theAcademy of Europe (2015), 6 prestigious fellowships in 6 countries,several patents, 3 monographs, and 470 journal papers. His researchon nanoscale control of energy and matter contributes to thesolution of the grand challenge of directing energy and matter atnanoscales, to develop renewable energy and energy-efficienttechnologies for a sustainable future.

Mahendra K. Sunkara currently directs the Conn Center forRenewable Energy Research and is a Distinguished UniversityScholar and a Professor of Chemical Engineering at University ofLouisville, U.S.A. He is an expert in the field of plasma-basedmaterials processing, large single-crystal growth, and nanowires forLi-ion batteries, solar cells, heterogeneous catalysis, electrocatalysts,and solar fuels. He has published over 120 peer-reviewedpublications and a book and has over 15 U.S. patents. He foundeda company, Advanced Energy Materials, LLC, which has scaled-upnanowire production for high performance heterogeneous catalystapplications. He received the distinguished faculty award for researchfrom University of Louisville and United Phosphorus DistinguishedSpeaker Award from the Indian Institute of Chemical Engineers inDecember 2009.

Annemie Bogaerts is full professor and Francqui DistinguishedResearch Professor at the University of Antwerp, Belgium. She is aworld-leading expert in modeling of reactive plasmas, with specialfocus on plasma catalysis for CO2 conversion and plasma medicineapplications. She has written over 350 peer-reviewed publications,including 11 invited reviews, as well as 10 invited book chapters. Shehas given over 100 invited lectures at international conferences and

has obtained 20 prestigious awards in various countries. She iscurrently also editor of Spectrochimica Acta: Part B and has oftenserved as guest editor for several journals, including a special issue inJ. Phys. D: Appl. Phys. (2014) on “Fundamentals of Plasma−SurfaceInteractions”.

ACKNOWLEDGMENTS

ECN and AB gratefully acknowledge financial support fromthe Fund of Scientific Research Flanders (FWO), Belgium,Grant Number G.0217.14N. KO acknowledges partial supportby the Australian Research Council and CSIRO’s OCEScience Leaders Program. MKS acknowledges partial supportfrom US National Science Foundation through grants DMS1125909 and EPSCoR 1355448 and also PhD studentsBabajide Ajayi, Apolo Nambo and Maria Carreon for theirhelp.

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Plasma catalysis: synergistic effects at the nanoscale. · 2015. 12. 27. · Plasma Catalysis: Synergistic Eﬀects at the Nanoscale Erik C. Neyts,*,§ Kostya (Ken) Ostrikov,†,‡